Application: Add set_accels_for_action(), deprecating add_accelerator().

* gtk/src/application.[hg|ccg]: Deprecate add_accelerator() and
  remove_accelerator() because they have been deprecated in the
  latest unstable GTK+ code:
  See https://bugzilla.gnome.org/show_bug.cgi?id=708905#c8
  Add set_accels_for_action(), set_accel_for_action() and
  unset_accels_for_action() to replace them, using the new
  gtk_application_set_accels_for_action() function.