This patch changes all uses of GDK_DEPRECATED(_FOR) in gtk headers
by the versioned variants, GDK_DEPRECATED_IN_3_x(_FOR). At the same
time, we add GDK_AVAILABLE_IN_3_x annotations for all API additions
in 3.2 and 3.4.
This is a GtkWindow subclass that "application windows" will use. Each
is associated with a GtkApplication, has the ability to show menus and
will have its own associated set of actions.