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/ViewProvider.cpp

@@ -68,6 +68,24 @@ using namespace std;
 using namespace Gui;
 
 
+namespace Gui {
+
+void coinRemoveAllChildren(SoGroup *group) {
+    if(!group)
+        return;
+    int count = group->getNumChildren();
+    if(!count)
+        return;
+    FC_TRACE("coin remove all children " << count);
+    SbBool autonotify = group->enableNotify(FALSE);
+    for(;count>0;--count)
+        group->removeChild(count-1);
+    group->enableNotify(autonotify);
+    group->touch();
+}
+
+} // namespace Gui
+
 //**************************************************************************
 //**************************************************************************
 // ViewProvider