These were not being used by QGIS code (always left at default 1.0
value), and removing them from the api allows us to simplify lots
of code. It also makes QgsRenderContext scaling much less confusing.
Switch the labeling gui to use this widget, which has the benefits:
- previews all label settings, including shadow and background
- previews at a specified scale, so that any sizes using map units
will be correct
