Option to keep projects sorted

[jeremypw]

Fixes #568

• Convert ACTION_ORDER_FOLDERS into a toggle action
• Convert "Manage Project Files menu child into a popover
• Use ModelButton for "Alphabetize" action
• Modify Fuzzy Finder plugin to add a button with accel label to the project menu
• Add a "order-folders" setting for persistence

Ordering the project folders in SourceList is not very efficient but as there are relatively few and that widget may well be replaced in Gtk4 it does not seem worth doing anything more complicated.

BEFORE

AFTER