[FEATURE] Add background shapes and svg symbols to labels

- Rectangle, square, circle and ellipse generated symbols
- SVG background via new symbology, so uses cache
- Add new gui class QgsSvgSelectorWidget, symbol selector setup culled from symbologyV2
- Does not include any associated data defined mappings, yet
- Caveat: does not currently account for background size in PAL collision calculations (overlaps occur)
- Allow direct update of QgsColorButton background