Breaking change on develop: MPE class updates

Heads up that in this commit on develop the MPE classes in JUCE have been updated to reflect the newly MMA-approved specification (see here for more details).

The biggest changes have occurred in the MPEZoneLayout class so if you have been writing software that deals with MPE zones you will need update accordingly. However, the higher-level classes such as MPEInstrument, MPESynthesiser, MPESynthesiserBase and
MPESynthesiserVoice should be unaffected.

The MPEDemo project in the JUCE examples folder is a good place to check out how these new changes have been implemented.