Gui: refactor tree view

* Major refactor of tree view to support external linking.

* Item update and selection change are now mostly handled by timer for
  performance improvement.

* Major change to drag and drop for better support of switching between
  copy, move and replace action, and auto adjustment of placement and
  relative link.

* Add second column for user changable object description.

* Unified tree view options and action into command group
  Std_TreeViewActions.

* Modified object search function to find external objects using
  Expression syntax.

src/Gui/ViewProviderDocumentObject.cpp

@@ -204,13 +204,21 @@ void ViewProviderDocumentObject::hide(void)
 
 void ViewProviderDocumentObject::show(void)
 {
+    if(TreeWidget::isObjectShowable(getObject()))
+        ViewProvider::show();
+    else {
+        Visibility.setValue(false);
+        if(getObject())
+            getObject()->Visibility.setValue(false);
+        return;
+    }
+
     // use this bit to check whether 'Visibility' must be adjusted
     if (Visibility.testStatus(App::Property::User2) == false) {
         Visibility.setStatus(App::Property::User2, true);
         Visibility.setValue(true);
         Visibility.setStatus(App::Property::User2, false);
     }
-    ViewProvider::show();
 }
 
 void ViewProviderDocumentObject::updateView()