clip: only allow core elements as children

clip: only allow core elements as children

Only allow elements that were created by ges_clip_create_track_elements
(or copied from such an element) to be added to a clip. This prevents
users from adding arbitrary elements to a clip.

As an exception, a user can add GESBaseEffects to clips whose class
supports it, i.e. to a GESSourceClip and a GESBaseEffectClip.

This change also introduces a distinction between the core elements of a
clip (created by ges_clip_create_track_elements) and non-core elements
(currently, only GESBaseEffects, for some classes). In particular,
GESBaseEffectClip will now distinguish between its core elements and
effects added by the user. This means that the core elements will always
have the lowest priority, and will not be listed as top effects. This is
desirable because it brings the behaviour of GESBaseEffectClip in line
with other clip types.