Usability improvements

[zykure]

Improve usability by adding two features:

• Middle-click on knobs and buttons resets them to default values. (This made it necessary to create deep-copies of used arrays/maps in the constructor.)

• Middle-click on patch panel ports removes existing connections. This works in addition to the "rewire-connection-to-remove" feature. In case of multiple connections on one port, the oldest one is removed first. The connection array now uses consistent order of its from/to fields to match actual input/output ports.