Before the change the compiler raised the warning: 'Gui::ViewProviderDatum::getRoot' hides overloaded virtual function [-Werror,-Woverloaded-virtual]
In the base class the method getRoot() is declared as const while in ViewProviderDatum it's not and that's why it's considered as an overloaded method.
However, this signature causes two problems:
1. In the client code it's not always clear which version of getRoot() should be called
2. It violates const-correctness
So, trying to declare the method ViewProviderDatum::getRoot() as const it now overrides the method of the base class (and fixes the warning) but
this doesn't lead to the expected result: See https://forum.freecad.org/viewtopic.php?p=796064#p796064
The other option is to rename the method. And this gives the expected result now.
Most of App::Origin is moved into this sub class of App::Origin.
Add App::Point. Change graphics of the planes/axis.
Remove scale-by-content behavior and make it fixed size on screen.
