Use GAction for file menu items
Use GAction for some view menu items

The sorting items remain.
Use GAction for file sorting mode GSetting
Use GMenuModel for popup menus
Remove now-unused UIManager proxy logic

It was used for showing tooltips of menu items in the status bar, but
GActions do not have tooltips so it is now useless.
Use GAction for toolbar stop button
Use named icons instead of GtkStock in some places
Convert XPM image data to PNG

Build PNG images into the easytag binary with GResource.
Avoid deprecated margin property with GTK+ 3.12
Refactor popup menu handling

Use gdk_event_triggers_context_menu() in button-press-event handlers.
Use GDK_BUTTON_PRIMARY instead of 1 where appropriate.
Remove last uses of GtkAction