Gui: refactor link property editor

PropertyLinkItem now support all major types of link property.
PropertyLinkListItem is no longer necessary, but kept for backward
compatible, which is now identical to PropertyLinkItem.

DlgPropertyLink, the link selection dialog, is now made modeless, so
that that user can select geometry sub-element directory from 3D view.

src/Gui/View3DInventorViewer.cpp

@@ -861,7 +861,7 @@ void View3DInventorViewer::checkGroupOnTop(const SelectionChanges &Reason) {
             return;
         }
         if(childRoot->findChild(childVp->getRoot())<0) {
-            FC_WARN("cannot find '" << childVp->getObject()->getFullName() 
+            FC_LOG("cannot find '" << childVp->getObject()->getFullName() 
                     << "' in geo group '" << grp->getNameInDocument() << "'");
             break;
         }