Clean up composer handling on application quit

Rather than a convoluted systems of signals and callbacks to be able to
prompt if unsaved composers should be closed on application quit, just
do so directly from GearyApplication::quit, and close any still-open
composers on controller shutdown.

This requires some more complicated state handling in the composer, but
allows greatly simplifiying the processes to both quit the application
and close th controller.
5 jobs for mjog/493-undo-send in 32 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
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passed #501844
fedora

00:08:34

passed #501845
ubuntu

00:14:14

 
  Test
passed #501846
flatpak

00:16:17

 
  Deploy
passed #501847
review

00:01:11

passed #501848
allowed to fail manual
stop_review

00:00:51