Support editing of audio-only files

[Arnei]

Currently we only render tracks with a video stream in our UIs. Tracks that have only an audio-stream are explicitly ignored.
opencast/opencast#5856 added support for audio-only events to Opencast. Opening the editor for such an event results in half-broken UI. I think the editor frontend should follow suit and allow proper editing of these events.

This would adapting the editor holistically. Each view should be checked separately if it could work with audio-only events, and what changes might be necessary. I'm assuming that the thumbnail and the track view in particular will require additional consideration.

We should also consider if it possible that the editor receives audio files which we do not actually want to display. Maybe because it is common to add them to the same flavor as the video tracks for Opencast workflow purposes, or something along those lines.

[Arnei]

A stop-gap measure would be to display a simple error screen for audio-only events, with a message along the lines of "the editor does not support audio files yet".