Cleaning some Skin - Control connection issues.

I have fixed some issues with Skin-Control connections.
See Commits.

This is a branch, on the way to remove the need for all connections parameters like <EmitOnPressAndRelease>true</EmitOnPressAndRelease>
Mixxx should always pick the right connection preferences by default, since it knows the type of Control it is about to connect.
This will make skin.xml a lot of smaller and much easier to modify. 
The connection preferences should remain as option only for special cases.

This branch is the first step.