Speed rename

This fixes Bug https://bugs.launchpad.net/mixxx/+bug/1283190
If you take "Speed  + up" like "Noun + verb" separate you get:  
"Geschwindigkeit erhöhen" 
"Geschwindigkeit reduzieren" 

If you take "Speed  + up" as single verb expression, you get:  
"beschleunigen"
"verlangsamen"

Just "beschleunigen" would be ambiguous sinc you can speed up the Speed and/or the tempo 
Since we desided for Speed = Geschwindigkeit as the noun, I think the first on might fit best.
In English it is almost the same: Speed up the tempo v.s. speed up the speed.  

Other thoughts?