Gui: add coinRemoveAllChildren to work around Coin3D bug

See bug description:
https://bitbucket.org/Coin3D/coin/pull-requests/119/fix-sochildlist-auditing/diff

Because of path based rendering (SoFCPathAnnotation) in mouse over
highlight, this bug causes crash more frequently here comparing to
upstream.

All C++ calling of SoGroup::removeAllChildren() is replaced by
Gui::coinRemoveAllChildren(), and python code is fixed by monkey
patching SoGroup.removeAllChildren() in FreeCADGuiInit.py.

src/Mod/Fem/Gui/ViewProviderFemConstraintPlaneRotation.cpp

@@ -119,7 +119,7 @@ void ViewProviderFemConstraintPlaneRotation::updateData(const App::Property* pro
         std::vector<Base::Vector3d>::const_iterator n = normals.begin();
 
         // Points and Normals are always updated together
-        pShapeSep->removeAllChildren();
+        Gui::coinRemoveAllChildren(pShapeSep);
 
         for (std::vector<Base::Vector3d>::const_iterator p = points.begin(); p != points.end(); p++) {
             //Define base and normal directions