Multiple input MIDI port support

Hi Juce Team,

I was wondering how keen you would be to support multiple MIDI input port in supported format (AAX, …).
In order not to change the juce::AudioProcessor callback signature, maybe the input port could be added in MidiMessage.
The struct won’t be bigger most of the time because the small size optimisation include 8 bytes so plenty enough to put regular MIDI message with the input port.

Thanks !