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/ViewProviderMirror.cpp

@@ -167,7 +167,7 @@ void ViewProviderMirror::unsetEdit(int ModNum)
         mf->Normal.setValue(norm[0],norm[1],norm[2]);
 
         pcRoot->removeChild(pcEditNode);
-        pcEditNode->removeAllChildren();
+        Gui::coinRemoveAllChildren(pcEditNode);
     }
     else {
         ViewProviderPart::unsetEdit(ModNum);