[GUI] Radial gradient implementation

This commit implements radial gradient as background and adds the option
to settings. It also renames "Color gradient" as "Linear gradient",
keeping Linear gradient default. Internally, it remains unchanged for
compatibility.

Radio gradient is more suitable for CAD, since it gives a more balanced
color distribution across the screen, improving visibility of model and
sketches with a halo-like effect.

src/Gui/View3DViewerPy.cpp

@@ -437,7 +437,7 @@ Py::Object View3DInventorViewerPy::setBackgroundColor(const Py::Tuple& args)
     }
     try {
         SbColor col(r,g,b);
-        _viewer->setGradientBackgroundColor(col,col);
+        _viewer->setGradientBackgroundColor(col, col, false);
         return Py::None();
     }
     catch (const Base::Exception& e) {