Gio::Action: Add templated change_state() and activate().

* gio/src/action.hg: Also adding change_state_variant() and
  activate_variant() for them to call, and deprecating the
  change_state() and activate() methods that take VariantBase.
  Also added change_state() and activate() templated methods
  that take Variant<T_Value> to preserve API for application
  code that previously called change_state(VariantBase) with
  a Variant<>.