Gui: Placement/DatumCS view provider changes

* 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.

src/App/Placement.cpp

@@ -51,6 +51,14 @@ Placement::~Placement(void)
 
 
 
+// Python feature ---------------------------------------------------------
+namespace App {
+PROPERTY_SOURCE_TEMPLATE(App::PlacementPython, App::Placement)
+template<> const char* App::PlacementPython::getViewProviderName(void) const {
+   return "Gui::ViewProviderPlacementPython";
+}
+template class AppExport FeaturePythonT<App::Placement>;
+}