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/ViewProviderFemConstraintPulley.cpp

@@ -114,7 +114,7 @@ void ViewProviderFemConstraintPulley::updateData(const App::Property* prop)
     if (strcmp(prop->getName(),"BasePoint") == 0) {
         if (pcConstraint->Height.getValue() > Precision::Confusion()) {
             // Remove and recreate the symbol
-            pShapeSep->removeAllChildren();
+            Gui::coinRemoveAllChildren(pShapeSep);
 
             // This should always point outside of the cylinder
             Base::Vector3d base = pcConstraint->BasePoint.getValue();