Gtk4Prep: ProjectChooserButton

[jeremypw]

• Use Controller instead of button_release_event
• Define action and target on each listboxrow instead of deprecated get_action_group/activate_action
• Do not listen to listbox remove signal - the GitManager is source of truth and we already listen to changes there. There is no way to remove rows (projects) through the UI of this widget.

This should work in Gtk4 with minimal changes for porting purposes.

There are no dead areas when clicking on the listboxrows.