Port from deprecated Gtk menu classes
requested to merge michaelweghorn/accerciser:michaelweghorn/port_from_deprecated_menu_classes into master
GtkAction GtkUIManager etc. were deprecated in Gtk 3.10.
Port the menu bar and the treeview context menu to use the recommended classes/interfaces like GMenu, GMenuModel and GAction instead.
See the individual commit messages for more details.