clip: incorrect `timeline` of children
Currently, you can add track elements to a clip from a different timeline. This can lead to a difference in the timeline of the clip and its children:
>>> timeline = GES.Timeline.new_audio_video()
>>> alt_timeline = GES.Timeline.new_audio_video()
>>> layer = timeline.append_layer()
>>> asset = GES.Asset.request(GES.TestClip, None)
>>> clip = layer.add_asset(asset, 0, 0, 10, GES.TrackType.AUDIO)
>>> effect = GES.Effect.new("agingtv")
>>> alt_timeline.get_tracks()[0].add_element(effect)
True
>>> clip.add(effect)
True
>>> clip.timeline is effect.timeline
FalseI propose that you should only be able to add a track element to a clip if it has the same timeline, or has a NULL timeline (and the clip/timeline should make sure to set the timeline once the element is added to a track).
Related: #87