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/Gui/Document.cpp

@@ -1559,7 +1559,7 @@ void Document::handleChildren3D(ViewProvider* viewProvider)
         if (childGroup->getNumChildren() != static_cast<int>(children.size())) {
 
             rebuild = true;
-            childGroup->removeAllChildren();
+            Gui::coinRemoveAllChildren(childGroup);
 
             for (std::vector<App::DocumentObject*>::iterator it=children.begin();it!=children.end();++it) {
                 ViewProvider* ChildViewProvider = getViewProvider(*it);