Use separate color config for RGB waveform

This fixes bug https://bugs.launchpad.net/mixxx/+bug/1376989
RGB Waveform: Set line width for drawing to 2.0
Minor speed-up in GLWaveformRendererRGB

Use three qreal color components rather than one QColor.
Merge pull request #363 from ninomp/rgb_improvements

Various improvements of RGB waveform