JUCE handling of MIDI plugins

Nice work reuk. Thanks! I think it would be also a good opportunity to add the recently introduced AAX_EPlugInCategory_MIDIEffect AAX category and address the interaction of JucePlugin_IsMidiEffect with the AAX code since paradoxically it has to be set to false for AAX Midi Effects to work (see AAX MIDI FX plugin via CMake detected as Instrument in Pro Tools).