Fix some const string warnings

Found with -Wwrite-strings and -Wcast-qual.
Add an app menu

Install application-wide actions for help, about and quit. Load the app
menu from a resource.
Add skeleton menubar from GtkBuilder file

The menu items are still not linked to actions, so do nothing when
clicked.
Use GAction for go-directory actions
Use GAction for miscellaneous menu items

Move action state update function to EtApplicationWindow.
Add toolbar for new GAction-using toolbar buttons
Use GAction for go-file actions
Use GAction for browser menu items
Use GAction for edit menu items
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