Comping: Auto-crossfade audio chunks

When comping several takes into one there is the risk of clicking/popping in the transitions. To avoid that there should be a very short auto-crossfade between the chunks.

Additionally, those crossfades should be fully editable (resize, move, select x-fade type), allowing to make smooth transitions e.g. through breaths or vowels.

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Posted by ~mylofy in ~alextee/zrythm-feature#423