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

This reverts commit 806031da.

Hide Composition's category in list view since transitions are not implemented yet

Fix in the refactoring branch
BUG: 398787

When dropping a folder on the bin, import only interesting items (video/audio/image/mlt playlist) and also import subfolders if they contain a clip

New 'Grab item' feature (Shift+G) allowing to select the current timeline item and move it in timeline with keyboard arrows (wip)