Add some more MIDI device slots to the MIDI prefs. Fixes #223 and #354.
Also, while we're at it, I'd like to fix the long-standing issue of beefing up the number of MIDI devices, which I promised to look into a long time ago. :) This PR makes 10 input and 10 output devices available, so we have one more than vanilla now. Should be enough? In the dialog, they're lined up in two columns now, 1-5 on the left, 6-10 on the right.