Make performance directory switchable

[probonopd]

With so many performances to choose from, we need a way to switch between multiple directories (folders) of performances.

Intended behavior:

• We can have performance/000000_factory/, performance/000001_somefolder/, performance/000002_someotherfolder/instead of a hardcodedperformance/` directory
• By sending the MIDI Bank Change messages on the MIDI channel configured for selecting performances (rather than voices), the directory can be switched
• It can also be switched in the UI

TODO:

• The changes in performanceconfig.* need to be reviewed; in CPerformanceConfig::SetPerformanceDir the performances from the new performance directory need to be loaded
• The changes in src/mididevice.cpp are pseudo-code and need to be implemented properly
• The Performances menu entry should get a new submenu to change the performance directory without using MIDI bank change messages

This PR is currently not functional because the TODOs need to be implemented. Any help appreciated. Volunteers?

[probonopd]

Closing in favor of @diyelectromusic's solution that has been merged.