Changed ControlObjectThreadMain instances to ControlObjectThread where no direct connection is used.

According to our previous discussion ControlObjectThreadMain ensures that the valueChanged signals are propagated by the Main Thread. This makes a different, when Qt::DirectConnection is used in the using code. 

I have checked all occurrences of ControlObjectThreadMain and changed them to ControlObjectThread if no direct connection is used. Most of them does not use any connection. So we save a unnecessary round trip thought the Qt event queue. 

If my other refactoring branch  https://github.com/mixxxdj/mixxx/pull/38 is accepted, we can change most (or all) of them to ControlObjectSlave. This will save some direct connected function calls ass well because it is optimized for no connection.