Random wave for instrument plugin LFO

Issue #750

This is a realisation of a random wave for the Instrument plugin LFO. It's not a general wave in Oscillator.cpp but calls Oscillator::noiseSample() to update the wave at intervals as taken from Vesas Monstro. The GUI representation is generated separately and is only a hint but it should be a decent representation of the timeframe. I checked it for backward compatibility and it seem to be up to no bad.
I looked at squashing the commits but it looked a bit dangerous so unfortunately it's not as clean a pull request as I wanted but here goes...  :-)
Coach: Diizy