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