* Major update to item drag in timeline
* Fix composition move in groups
* Refactor timeline preview (in progress)

This reverts commit 806031da.

Introduce split video, fix issues with audio split

Test Plan:
- Add clips to the project bin
- Select two or more clips
- Drag them to the timeline
- Check that the "Insert Clips" entry got added to the undo history
- Undo the change
- Redo the change

Reviewers: alcinos, mardelle

Reviewed By: alcinos, mardelle

Differential Revision: https://phabricator.kde.org/D8928

Reintroduce automatic track for compositions
Remove buggy track indicator for compositions

Start using target tracks for timeline lift/extract/insert

Use a new copy of the bin producer on each insertion in timeline to allow audio only / video only clips and avoid issues on audio mix

Has to be enabled through button in effectstack view

Move clip cut in separate file to clean up timelinemodel. Don't reset timeline id on project opening

Fix monitor switching sometimes leaving monitor blank