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/CombiView.cpp

@@ -63,8 +63,7 @@ CombiView::CombiView(Gui::Document* pcDocument, QWidget *parent)
     QSplitter *splitter = new QSplitter();
     splitter->setOrientation(Qt::Vertical);
 
-    // tree widget
-    tree =  new TreePanel(this);
+    tree =  new TreePanel("ComboView", this);
     splitter->addWidget(tree);
 
     // property view