Gtk4Prep: Use ListStore in ProjectChooserButton - #1787

• Avoid listbox.get_children () which goes away in Gtk4
• Re-use model in GitManager to build ListBox
• Apply filter with a filter func
• Simplify ProjectRow
• Update active project when popover shown (and at start)
• Close popover when active project changed so that the row states will be updated on reopening

The project listbox is now built from a pre-existing model containing objects (ProjectFolderItems).