LibGUI: Allow dragging a previously unselected item with a single click

Views would previously require that an item be selected before it could
be dragged. This patch makes us consider initiating a drag immediately
after the view has been selected, without requiring a mouseup event in
between.

Libraries/LibGUI/AbstractView.cpp

@@ -235,6 +235,7 @@ void AbstractView::mousedown_event(MouseEvent& event)
         set_cursor(index, SelectionUpdate::ClearIfNotSelected);
     } else {
         set_cursor(index, SelectionUpdate::Set);
+        m_might_drag = true;
     }
 
     update();