Pitch scale

This is the fist chunk of commits, that fixes the scale jump issue of the pitch knob. 
https://bugs.launchpad.net/mixxx/+bug/1258617
The code is still a bit ugly because it still uses the old pitch adjust CO internally but it works. 
I am working on a solution where we just pass pitch and tempo to the scalers, which will clean up the code.
The new track will be pitched the same way as the old track. It is the responsibility to tweak the sliders on the new track as it is a stomp box effect that does not know anything about the track change.  If he turns the pitch knob back to center, it will be at the linear pitch. 
If this is not desired, he can enable the "Reset speed and pitch on track load" option in preferences. (Which will be my default choice, but we have decide on to reset for Mixxx default)