LibGUI: Move selection behavior from TableView up to AbstractView

Let's make SelectionBehavior a view concept where views can either
select individual items (row, index) or whole rows. Maybe some day
we'll do whole columns, but I don't think we need that now.

Libraries/LibGUI/AbstractTableView.cpp

@@ -41,6 +41,7 @@ namespace GUI {
 
 AbstractTableView::AbstractTableView()
 {
+    set_selection_behavior(SelectionBehavior::SelectRows);
     m_corner_button = add<Button>();
     m_corner_button->move_to_back();
     m_corner_button->set_background_role(Gfx::ColorRole::ThreedShadow1);