* lisp/system-taskbar.el (system-taskbar--set-back-end): Add
boundp condition for w32-initialized.
(system-taskbar--enable): Add success return value to all
methods.  Demote error to warning in the dbus method.

* src/w32fns.c (Fw32_badge): Fix typos in specifying color values.

* lisp/system-taskbar.el (system-taskbar-w32-badge-background)
(system-taskbar-w32-badge-foreground): Adjust to Windows
conventions.  (Bug#79859)

* doc/emacs/frames.texi:
* doc/lispref/os.texi:
* lisp/system-taskbar.el: Fix typos.

Implement system GUI taskbar/dock/launcher icon badge, icon
progress indicator, icon attention alert features for D-Bus
platforms (tested on KDE and GNOME), NS (macOS/GNUstep),
MS-Windows.

Add 'progress-reporter-update-functions' abnormal hook to facilitate
taskbar progress display, and other custom progress reporters.
The default function list is 'progress-reporter-echo-area' which
is backward compatible.

* lisp/subr.el (progress-reporter-update-functions):
New defvar.
(progress-reporter-echo-area): New defun.
(progress-reporter-do-update): Run
progress-reporter-update-functions for both numerical and
pulsing reporters.
(progress-reporter-done): Run progress-reporter-done-functions.
* lisp/system-taskbar.el: New file.
* src/nsfns.m (Fns_badge, Fns_progress_indicator)
(Fns_request_user_attention): New function.
(syms_of_nsfns): Add defsubr Sns_badge,
Sns_request_user_attention, Sns_progress_indicator. Add DEFSYM
Qinformational, Qcritical.
* src/w32fns.c (rgb_list_to_colorref, Fw32_badge)
(Fw32_request_user_attention, Fw32_progress_indicator): New
function.
(syms_of_w32fns): Add defsubr Sw32_badge,
Sw32_progress_indicator, Sw32_request_user_attention. Add DEFSYM
Qinformational, Qcritical.
* doc/emacs/frames.texi: User documentation.
* doc/lispref/os.texi: Programmer documentation.
* etc/NEWS: Announce system-taskbar-mode.  Announce progress
reporter callback enhancements.