* fix(overlay): quit Vocalinux when the dock closes the overlay window (#1028)
On GNOME Wayland without layer shell, the prewarmed dictation overlay is a
regular toplevel that the dock lists as the app's window. Since #1017 the
overlay refuses close requests so it is never destroyed (#1010), which made
"Quit" from the dock silently do nothing.
The overlay still keeps its window, but now forwards the close request to
an optional on_close_request callback, scheduled once from an idle source.
The tray passes its _quit(), so the dock's Quit behaves like the tray's Quit.
* fix(overlay): drop a queued dock close once the app is quitting
A dock close request is forwarded through an idle source. If the tray's
Quit or SIGTERM tears the overlay down before that idle runs, the
callback would run _quit() a second time. destroy() now drops the
callback, and the pending flag is cleared when the request runs so a
later close can retry if quitting failed.
Refs #1028
