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/ViewProviderInventorObject.cpp

@@ -95,7 +95,7 @@ void ViewProviderInventorObject::updateData(const App::Property* prop)
         // read from buffer
         SoInput in;
         std::string buffer = ivObj->Buffer.getValue();
-        pcBuffer->removeAllChildren();
+        coinRemoveAllChildren(pcBuffer);
         if (buffer.empty()) return;
         in.setBuffer((void *)buffer.c_str(), buffer.size());
         SoSeparator * node = SoDB::readAll(&in);
@@ -112,7 +112,7 @@ void ViewProviderInventorObject::updateData(const App::Property* prop)
         QString fn = QString::fromUtf8(filename);
         QFile file(fn);
         SoInput in;
-        pcFile->removeAllChildren();
+        coinRemoveAllChildren(pcFile);
         if (!fn.isEmpty() && file.open(QFile::ReadOnly)) {
             QByteArray buffer = file.readAll();
             in.setBuffer((void *)buffer.constData(), buffer.length());