VuMeter bar to use full widget size

Check widget size to know bar size instead of checking bitmap size to allow resizable VuMeters.
Also suddenly found fix for unavailable horizontal VuMeter peaks.
P.S. I don't know C++ at all, so code may be "incorrenct", but it works with all skins, just tested.
Pretty sure this needs to be converted to use ```Paintable::draw(QRect, QPainter, QRect)``` -- using a target point will use the sourceRect's size.