* Add always zoom invariant and always on top rendering to
ViewProviderDatumCS
* Add a new utility class AxisOrigin for more efficient axis rendering.
Also exposed to python.
* Change ViewProviderPlacement to use AxisOrigin for rendering.
A single instance of AxisOrigin is shared by all
ViewProviderPlacement. Selection context is used to distinguish among
different instances.
- Make Datums colorable and transparent
- Change default datums color to yellow and transparency to 60%
- Rework Lines and Points datum view provider
- Add ViewProviderDatum::defaultBoundBox()
