actor: Add CLUTTER_REQUEST_CONTENT_SIZE mode

Some actors want to have a preferred size driven by their content, not
by their children or by their fixed size.

In order to achieve that, we can extend the ClutterRequestMode
enumeration so that clutter_actor_get_preferred_size() defers to the
ClutterContent's own preferred size.

https://bugzilla.gnome.org/show_bug.cgi?id=676326