Gio::MenuItem: get/set_attribute(): Deprecated for *_value() methods.

* gio/src/menuitem.hg: set_attribute(): Deprecate this in favour of
  set_attribute_value() though it was previously the other way around.
  Add get_attribute_value() and deprecate get_attribute() in favour
  of it.
  This lets us, one day, remove those set_attribute() and
  get_attribute() methods and replace them with more pleaseant
  templated set/get_attribute() methods that just call the
  *_value() methods, as in Gio::Action.