Better widget image scaling support.

* Add STRETCH_ASPECT and FIXED DrawModes.
* Guard (almost) all setFixedSize calls in widgets on FIXED DrawMode.
* Remove WStatusLight SizeMode since it was added to allow side-stepping setFixedSize calls.
* Remove pixmap fall-back for SVGs when rendering with a source rectangle (i.e. VU meters). I finally got this working with QSvgRenderer.
* De-dupe logic in Paintable::draw methods so they all funnel through one handler -- drawInternal.
* Use floating point for rectangle calculations in WVuMeter instead of integers.
* SkinContext helper for extracting scalemode.
* Support for "scalemode" attribute on (almost) all widget images.

The only remaining widget that doesn't have scalemode support and still calls setFixedSize is WSpinny -- which uses QImage's instead of Paintables.
Well, DrawMode is a totally Mixxx thing -- not Qt. 

I agree though -- but that ship has sailed... since our new UI system is essentially a giant pile of QLayouts -- you kind of have to know about how Qt does layout .. which means understanding sizemodes, sizeincrements, minimumsize, maximumsize, etc.