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.

samples/listctrl/listtest.h

@@ -61,6 +61,7 @@ public:
     void OnListKeyDown(wxListEvent& event);
     void OnActivated(wxListEvent& event);
     void OnFocused(wxListEvent& event);
+    void OnItemRightClick(wxListEvent& event);
     void OnCacheHint(wxListEvent& event);
 
     void OnChar(wxKeyEvent& event);