In this new version I changed the way events are handled.
Now llk-handle generates input events to be used with normal
keymaps, instead of running a command. The function llk-bind
activates event generation for a key and a combination of
events press, release, double, triple.

I also made changes following your notes, such as:

- Clarification on the event's time, documentation of
  keysyms variables moved, don't limit to GUI systems.
- cl-defstruct for the event payload.
- Format and naming conventions.
- Default timeout to mouse double click timeout.

You suggested to auto generate the keysym table, but we
would still need a table to relate the Windows keys to the X
equivalent.

* doc/lispref/commands.texi (Misc Events): Adjust to new semantics.
* src/keyboard.c (syms_of_keyboard): Adust docstring.

FIXME: Old commit msg.

* lisp/low-level-key.el (New file).
(low-level-key): Struct with event data.
(llk-bindings): User bindings for low level key events.
(llk-tap-timeout): User option.
(llk-keysyms): List of available keysyms.
(llk--define-xk): Macro for defining keysyms.
(llk--define-keysyms): Build llk-keysyms.
(llk-bind): Function to create a binding.
(llk--event-history-for-tap): Event history for tap detection.
(llk--detect-n-tap): Function to detect taps.
(describe-low-level-key): Command to get information about a key.
(llk--describe): Show help buffer with information
about an event.
(llk-handle): Handler for key events.
(llk--generate-event): Generate input event for key.
(llk--generate-events): Generate input events for key.

* doc/lispref/commands.texi (Misc Events): Improve style and clarify
the dependence on `enable-low-level-key-events`.
* src/keyboard.c (syms_of_keyboard): Fix markup.

* doc/lispref/commands.texi: Added information about new event.
* src/keyboard.c (syms_of_keyboard): Tweak docstring.

FIXME: Old commit msg.

* lisp/low-level-key.el (New file).
(llk-bindings): User bindings for low level key events.
(llk-tap-count): User option.
(llk-tap-timeout): User option.
(llk-tap-keys): User option.
(llk-keysyms): List of available keysyms.
(define-xk): Macro for defining keysyms.
(llk-define-keysyms): Build llk-keysyms.
(llk-init): Function to initialize low level key handling.
(event-is-key-press): Get field from event.
(event-keysym): Get field from event.
(event-modifier): Get field from event.
(event-time): Get field from event.
(llk-bind): Function to create a binding.
(llk-events): Event history for tap detection.
(llk-detect-n-tap): Function to detect taps.
(describe-low-level-key): Command to get information about a key.
(llk-show-event-description): Show help buffer with information
about an event.
(llk-handle): Handler for key events.

Detect double/tripe taps.

[FIXME: Need adjustments in the description below!]

* src/gtkutil.c (xg_create_frame_widgets): Modified to handle key
release events.
(xg_maybe_send_low_level_key_event): New function that sends key
events.
(xg_widget_key_press_event_cb): Modified to send low level key events.
(xg_widget_key_release_event_cb):New function to send low level key
events on key release.
* src/keyboard.c (kbd_buffer_get_event): Modified to handle low level
keyboard events.
(make_lispy_event): Modified to handle low level keyboard events.
(kbd_low_level_key_is_enabled): New function to decice if a particular
key should generate events, looking at configuration,
(syms_of_keyboard): Added symbols and varialbles.
(keys_of_keyboard): Modified to map low level key events to
'special-event-map'.
* src/keyboard.h: Modified with function prototype.
* src/pgtkterm.c (pgtk_maybe_send_low_level_key_event): New function
that sends key events.
(key_press_event): Modified to handle low level keyboard events.
(key_release_event): Modified to handle low level keyboard events.
* src/termhooks.h: Modified to define constant.
* src/w32fns.c (w32_wnd_proc): Modified to generate low level key
events.
* src/w32term.c (w32_read_socket): Modified to generate low level key
events.
* src/xterm.c (x_get_modifier_for_keycode): New function to decide
which (if any) modifier corresponds to a given key.
(x_maybe_send_low_level_key_event): New function that sends key
events.
(handle_one_xevent): Modified to handle low level key events.
(syms_of_xterm): Modified to definde symbol.
* src/xterm.h: Modified with function prototype
* lisp/low-level-key.el (New file).
(llk-bindings): User bindings for low level key events.
(llk-tap-count): User option.
(llk-tap-timeout): User option.
(llk-tap-keys): User option.
(llk-keysyms): List of available keysyms.
(define-xk): Macro for defining keysyms.
(llk-define-keysyms): Build llk-keysyms.
(llk-init): Function to initialize low level key handling.
(event-is-key-press): Get field from event.
(event-keysym): Get field from event.
(event-modifier): Get field from event.
(event-time): Get field from event.
(llk-bind): Function to create a binding.
(llk-events): Event history for tap detection.
(llk-detect-n-tap): Function to detect taps.
(describe-low-level-key): Command to get information about a key.
(llk-show-event-description): Show help buffer with information
about an event.
(llk-handle): Handler for key events.

And detection of double/triple taps on modifier keys.

* lisp/low-level-key.el (llk-tap-timeout):
(llk-tap-count):
(llk-tap-keys):
(llk-bindings):
(llk-modifier-bindings):
(llk-init):
(llk-bind):
(llk-modifier-bind):
(llk-events):
(llk-modifier-events):
(llm-handle):
* src/gtkutil.c (xg_create_frame_widgets):
(xg_maybe_send_low_level_key_event):
(xg_widget_key_press_event_cb):
(xg_widget_key_release_event_cb):
* src/xterm.c (x_maybe_send_physical_key_event):
(x_filter_event):
(handle_one_xevent):
(syms_of_xterm):

* lisp/tab-bar.el (tab-bar--update-tab-bar-lines):
Revert '(not (daemonp))' to update 'default-frame-alist'
even in daemon mode (bug#78896).

* lisp/frame.el (frame-notice-user-settings): For daemon's
initial frame don't modify the frame parameter 'tab-bar-lines'.

* lisp/gnus/mm-decode.el (mm-inline-media-tests): Add "image/svg+xml".