[clone] Allow cloning unparented actors

[clone] Allow cloning unparented actors

If the source of a ClutterClone does not have a parent actor the
clone will be unable to paint it because it's missing an allocation.

A simple way to fix it is to make the ClutterClone act like a
"foster parent": when it is allocated it will check if the source
actor has a parent and if not it will allocate the source with
its preferred size.