altona: enumerate only physical MIDI outs

sAddMidi() can now optionally enumerate only physical MIDI outputs, sans
GM softsynth et al.
screems4: fixes, additions (midi)

- fixed crash when trying to play unsupported video files
- now the pending next slide gets refreshed from the server
automatically
- made asset refresh more resilient against http errors
- fixed occasional crash when starting videos
- added sending of MIDI notes when keys are pressed (for syncing
external hardware to the jingles, etc)
- more TUM2012 slides
screens4: tum2012 fixes/additions

* use separate renderer for fully opaque slides (no background fx)
* added prefix for transition ops to config file
* added "Epic Realtime" TUM2012 slide background
* added "drunk mode" slide renderers for extra wibbly wobbly