This fixes drop_action_unregister() to not call g_object_get_data()
on priv->stage if not yet resolved. This can happen if the action's
actor was destroyed before ever being mapped.
The drop-cancel signal allows the drop action to cleanup its
state if the can-drop signal is refused. This is especially
useful if the drop action (or its target actor) is managing
any drop target animations.
https://bugzilla.gnome.org/show_bug.cgi?id=675336
The example code that is meant to be XIncluded into the API reference
should not be part of the interactive test suite: it's code that it is
meant to be used as a reference implementation - whereas the interactive
test suite should be allowed to be lean and test behaviour even in nasty
ways. In short: the test suite should not be the place where we show off
idiomatic code for educational purposes.
