* 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.
Before that (e.g. on MacBook Pro) it was very hard to see the microscopic selection-circle hidden behind much bigger cursor arrow when trying to snap e.g. a line to some vertex. Now this setting can be used for such displays to increase the marker size and make it clearly visible behind the cursor.
The already existing "sketcher marker size" property is intended to be used for sketcher only, so it's not suitable for global setup of all the markers' sizes uniformly...
+ fix SoAutoZoomTranslation and SoZoomTranslation to get correct scaling factor
(do not rely on active view because this can be the wrong one and thus delivers wrong results)
-removes all soqtviewer references
-create a quarter adaptor class which implements missing functionality
-adopts freecad code to make use of SoRenderManager and changed View3DInventorViewer interface
