Speed rename

This fixes Bug https://bugs.launchpad.net/mixxx/+bug/1283190
My logic was Speed changes Musical Pitch and Musical tempo.
Except Keylock is enabled. Then it changes only the musical tempo. 
If we may have a tempolock, it changes only the pitch. 
From this point of view the "Pitch" label on a Turntable is only a Short form of "Pitch and tempo". 
Technically the should have lable it "Platter Speed".   
This is a no issue on turntables, because the can't control Pitch and tempo independently, but it is important in Mixxx.
For the menu title, I would like to keep the current version. But it is a good idea to reference the turntable pitch slider in the description below.