Don't generate wxEVT_LIST_ITEM_RIGHT_CLICK outside of item area

Make wxGenericListCtrl used under wxGTK and wxOSX behave the same as the
native wxMSW wxListCtrl and wxTreeCtrl under all platforms and only generate
ITEM_RIGHT_CLICK events when an item was actually clicked, i.e. don't generate
them for the clicks completely outside of the client area.

Closes #4711.

src/generic/listctrl.cpp

@@ -2432,8 +2432,6 @@ void wxListMainWindow::OnMouse( wxMouseEvent &event )
         // outside of any item
         if (event.RightDown())
         {
-            SendNotify( (size_t) -1, wxEVT_LIST_ITEM_RIGHT_CLICK, event.GetPosition() );
-
             wxContextMenuEvent evtCtx(
                 wxEVT_CONTEXT_MENU,
                 GetParent()->GetId(),