Currently copies behavior of clip and image editors:
- On file load, all strip will reference clip they're using
- On adding new strip, clip would be referenced only if it've got zero user
- On removing strip clip wouldn't be de-referenced to prevent clip editors
pointing to zero-counted datablocks.
Not actually ideal from human beings point of view, but referencing/dereferencing
clip on each strip add/delete is getting crappy because of current logic of how
clip datablocks are referenced from clip editor (which is designed to work fine with
loading files without loading UI).
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