Make virtual lists send DESELECTED events consistently on both implementations

In the case when a listctrl is clicked outside of any item, the item is
visually deselected, and it is logical that a DESELECTED is sent.

For non-virtual lists this would happen, but for virtual lists it would
either happen or not happen depending on implementation (MSW/generic)
and mode (single/multi). From now on the DESELECTED event is always sent.

src/generic/listctrl.cpp

@@ -2447,7 +2447,8 @@ void wxListMainWindow::OnMouse( wxMouseEvent &event )
     else
         m_dragCount = 0;
 
-    // The only mouse event that can be generated without any valid item is
+    // The only mouse events that can be generated without any valid item are
+    // wxEVT_LIST_ITEM_DESELECTED for virtual lists, and
     // wxEVT_LIST_ITEM_RIGHT_CLICK as it can be useful to have a global
     // popup menu for the list control itself which should be shown even when
     // the user clicks outside of any item.
@@ -2467,6 +2468,10 @@ void wxListMainWindow::OnMouse( wxMouseEvent &event )
         {
             // reset the selection and bail out
             HighlightAll(false);
+            // generate a DESELECTED event for
+            // virtual multi-selection lists
+            if ( IsVirtual() && !IsSingleSel() )
+                SendNotify( m_lineLastClicked, wxEVT_LIST_ITEM_DESELECTED );
         }
 
         return;