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.

interface/wx/listctrl.h

@@ -1442,7 +1442,7 @@ protected:
         control itself when this event is generated, see @ref
         overview_events_with_mouse_capture "event handling overview".
     @event{EVT_LIST_ITEM_DESELECTED(id, func)}
-        The item has been deselected.
+        The item has been deselected. GetIndex() may be -1 with virtual lists.
     @event{EVT_LIST_ITEM_ACTIVATED(id, func)}
         The item has been activated (ENTER or double click).
     @event{EVT_LIST_ITEM_FOCUSED(id, func)}