Make sure to allocate all children in allocate vfuncs

Clutter expects actors overriding the allocate vfunc to allocate all mapped children of the actor, otherwise bad things happen.

So make sure we actually allocate all our visible children in our custom allocation functions, and since we don't want to give them a real allocation, just pass them an empty ClutterActorBox.

It would be nice if we had a way to hide children during the allocation process where no relayout is queued like gtk allows with gtk_widget_set_child_visible(), then we could avoid those weird empty ClutterActorBoxes.

Fixes #3098 (closed)