clip: make sure core child is active for non-core in same track

clip: make sure core child is active for non-core in same track

Each active non-core child must have a corresponding active core child
in the same track. Therefore, if we de-activate a core child, we also
need to de-activate all the non-core children in the same track.
Similarly, if we activate a non-core child, we need to activate the
corresponding core child as well.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/169>