songwidget: Handle checkbox interactions with selection-mode

There is a bidirectional binding between the coresong selected
property and the checkbox active state. It keeps these properties
synchronized.
However, the selection change from the DiscBox is deactivated because
a selection change would be propagated to the checkbox which would
undo the selection.

Solve this issue by adding a flag which keeps track of the clicks and
prevents the same event from happening twice (one time in the checkbox
and again in the SongWidget).
4 jobs for wip/jfelder/songswidget-selection in 9 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #370501
flake8

00:01:40

 
  Build Flatpak
passed #370502
flatpak

00:05:32

 
  Deploy
passed #370503
review

00:01:02

passed #370504
allowed to fail manual
stop_review

00:00:59