voc2gui: Allow cutting between different Channel B cams in lecture composite

[schmittlauch]

From my experience, it can be helpful in emergencies (broken camera source, something getting in the way of the image) to quickly cut between different cameras in channel B (miniature). In the lecture composite, this currently is not possible while staying in that mode as no transition/ cut exists between the states. It is necessary to switch to full screen first.

how to reproduce

1. channel A = slides, channel B = cam 2
2. cut/ transition to make that the live state
3. retake
4. channel b = cam1

observation: in channel B all cameras are selectable (buttons are active), but the Cut or Trans buttons remain inactive and activating the composite is not possible. The only mix button becoming active when changing the channel B source is retake, but that is also behaving weirdly (see #297).

expected: The cut button shall be active, allowing a swift and immediate change of the small channel B's image in case of emergency.

alternative or broader approach

IMHO having buttons active and selectable, but after doing so not being able to actually "cut" or "trans" to that live state, breaks my mental state machine and is unexpected.
Thus, if actually not wanting to allow changing channel B in lecture mode by design, then all other channel B source buttons are supposed to be inactive in the situation described above.