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/ViewProviderFemConstraintFixed.cpp

@@ -143,7 +143,7 @@ void ViewProviderFemConstraintFixed::updateData(const App::Property* prop)
         int idx = 0;
 #else
         // Note: Points and Normals are always updated together
-        pShapeSep->removeAllChildren();
+        Gui::coinRemoveAllChildren(pShapeSep);
 #endif
 
         for (std::vector<Base::Vector3d>::const_iterator p = points.begin(); p != points.end(); p++) {