Add method ActionsRegistry.create_menu_item()

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Force show menubar if DIORITE_GUI_MODE is true

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Use MenuModel to specify gear menu

We get popover for free :-)

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>