Make JACK output ports dependent on general instrument property

[theGreatWhiteShark]

Right now the per instrument ports are associated with the position of the individual drumkits. When switching drumkits the output port of the topmost instrument in the new drumkit will replace the one of the topmost one in the old one. This, unfortunately, does almost never conserve the type of instrument associated with the port and messes up the wiring.

By basing the ports and the general instrument property in #1068 and using the mapping function when changing drumkits #1070 one could seamlessly switch between drumkit while using an external, JACK-based mixer.

[theGreatWhiteShark]

This will also fix #71