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/ViewProviderVRMLObject.cpp

@@ -216,7 +216,7 @@ void ViewProviderVRMLObject::updateData(const App::Property* prop)
         QString fn = QString::fromUtf8(filename);
         QFile file(fn);
         SoInput in;
-        pcVRML->removeAllChildren();
+        coinRemoveAllChildren(pcVRML);
         if (!fn.isEmpty() && file.open(QFile::ReadOnly)) {
             QFileInfo fi(fn);
             QByteArray filepath = fi.absolutePath().toUtf8();