* src/client/application/geary-application.vala (GearyApplication): Set
  the resource_base_path to comply with the expected modern app id, so
  the app menu will be automatically loaded.

* ui/CMakeLists.txt: Replace app_menu.interface with gtk/menus.ui. Don't
  try ot copy interface files now that there aren't any.

* gtk/menus.ui: Replaces app_menu.interface.