[RfC] Reconsider pre-sets

[danimo]

In the original design we wanted to void pre-sets. However, the following situation can easily become arkward:

Scenario: Camera on Channel A, Slides on Channel B.

1. Coming from Fullscreen Camera, I want to be able to cut to SBS Preview with Slides in the main window.

This means I must swap the A channel, then quickly switch over. This easily becomes confusing and slow as I must agree after using voctomix 2 again after two years. So we might indeed be better off with pre-sets. Depending on the event we can either have them pre-defined through the config files or allow the operator to pick them.

[jbeyerstedt]

In my point of view, voctomix should make video mixing as easy and mistake-resilient as possible to enable our workflow with volunteers who only get a short introduction to the video mixer. Therefore I would like to split this into two features:

1. When there's only one camera, there are only a few possible useful compositions: fs(slides), fs(cam1), lec(slides, cam1), sbs(slides, cam1). So four buttons (presets) would be enough.
2. (If there are more cameras,) the lecture mode should not allow the slides to be Channel B in the lecture mode as this never makes sense (because cannel B is cropped).