Application: add/remove_accelerator(): Add overloads without the variant parameter.
Application: add/remove_accelerator(): Add overloads without the variant parameter.

* gtk/src/application.hg: Use the {?} syntax to add methods that
  do not take the parameter.

GNOME/gtkmm,johne53/MB3Gtkmm,johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm