Allow MIDI messages to affect multiple controls.

[Bruno Buccolo]

Let's say a user wants to map one midi message to multiple controlobjects:

Button 1 --> Sync Deck A
Button 1 --> Sync Deck B

or

Button 1 --> Flanger Deck A
Button 1 --> Crossfader to Side A

Something like that is impossible to Mixxx, since after the first mapping is made, the second mapping of the macro overlapps the first one.

I'm working on the bug, currently trying to use QMultiHash for the MidiInputMapping, but it was really not as simple as I tought.
I'll post up the research as I go, and also link this bug to my Midi Learning Branch since I'm already working with midi there :)

Thanks.

== Followup ==

By using QHash::insertMulti or even QMultiHash for the MidiInputMapping, it seems that Mixxx can already save the correct midi.xml, however, it doesn't work (macro) and it's not displayed correctly in the MidiInputMappingTable.

[Bruno Buccolo (bruno-buccolo)]

It looks like this was already known since 2008:
http://www.mixxx.org/wiki/doku.php/gsoc2008_midi_control

[RJ Skerry-Ryan (rryan)]

This should be pretty straightforward to support at the XML level. The new mapping GUI should also support this when we add it back in.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/5395