actor: Correct setting the offscreen-redirect property

It's a flags property, not an enum one.

https://bugzilla.gnome.org/show_bug.cgi?id=708922
ClutterEvent: Mention _get_source_device() in docs

It's too easy getting bitten by the ->device red herring, thinking
that it's the original input device the event originated from.

https://bugzilla.gnome.org/show_bug.cgi?id=709620