clip: add support for time effects
Allow support for time effects being added to clips.
In particular, take time effects into account when:
- calculating the duration-limit
- splitting
- trimming
Also add method for specifically adding effects to a clip, that also checks for track selection errors.
Also added more error codes to GES_ERROR
related to timeline edits. This introduces a few _full()
variants, but is also used in the new methods.