Add scene selection

[fredemmott]

• [x] basic scene switching and listing
  • [x] OBS
  • [x] XSplit
  • [x] StreamDeck plugin
  • [x] WebUI
• [x] scene preview images
  • [x] OBS
  • [x] Relicense OBS plugin to GPLv2
  • [x] XSplit
  • [ ] StreamDeck plugin
  • [x] WebUI

[fredemmott]

Currently tested by making local changes to the web UI, but working in both XSplit and OBS

[fredemmott]

Caveat: XSplit will only provide a thumbnail if the scene has previously been activated

[fredemmott]

OBS also done, with similar caveat: many sources are inactive if not visible

[fredemmott]

Done, just artwork needed.