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.
Introduce a 'custom-item-activated' on the widget, which behaves
similairly to GtkEntryCompletion::action-activated, i.e. is emitted when
a custom item is chosen from the dropdown list.
Clients can use the name provided when adding the item as a detail for
the signal, to get notified when that specific item is activated, or use
the signal without details to get notifications for all custom items.
