LibGUI: Default TreeView to SelectionBehavior::SelectItems

AbstractTableView (which TreeView inherits from) sets the selection
behavior of the view to SelectRows. This is not how TreeViews are used
in most of the system, and TreeView::paint_event actually always draws
with the assumption of selecting individual items. This commit defines
the expected selection behavior for TreeViews. Users of TreeView can
still override this via TreeView::set_selection_behavior.

Userland/Libraries/LibGUI/TreeView.cpp

@@ -34,6 +34,7 @@ TreeView::MetadataForIndex& TreeView::ensure_metadata_for_index(const ModelIndex
 
 TreeView::TreeView()
 {
+    set_selection_behavior(SelectionBehavior::SelectItems);
     set_fill_with_background_color(true);
     set_background_role(ColorRole::Base);
     set_foreground_role(ColorRole::BaseText);