Refactor tracks and logic around subtracks

[christoph-fricke]

Currently, everything is contained in the MuttiTrackElement, which makes it harder to test the sub-track logic. Further, moving focus with the keyboard is achieved through event bubbling/mediators and until the element reaches a track that can handle it. Moving the focus could be done in one place if the timeline would store a data-structure of all tracks and their subtracks. However, it would make the tracks less independent.