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/Part/Gui/ViewProviderCurveNet.cpp

@@ -118,8 +118,8 @@ void ViewProviderCurveNet::updateData(const App::Property* prop)
         if (cShape.IsNull())
             return;
 
-        EdgeRoot->removeAllChildren();
-        VertexRoot->removeAllChildren();
+        Gui::coinRemoveAllChildren(EdgeRoot);
+        Gui::coinRemoveAllChildren(VertexRoot);
 
         try{
             computeEdges   (EdgeRoot,cShape);