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.
reset_key resets the pitch allays to the original track key, pitch_set_default resets the pitch slider, which can be the linear key or the original key, depending on the mode and the key-lock state.