Restructure wxNotificationMessage.

wxNotificationMessage has been refactored to always use wxNotificationMessageImpl (this was previously already done in the MSW implementation)

This adds various features and fixes to wxNotificationMessage:
- OS X Notification Center implementation
- Generic "toast" notifications
- SetIcon() to specify a custom icon
- AddAction() to add actions to notifications
- Events to get notify of notification clicks, dismiss or actions

include/wx/msw/taskbar.h

@@ -49,7 +49,8 @@ public:
     bool ShowBalloon(const wxString& title,
                      const wxString& text,
                      unsigned msec = 0,
-                     int flags = 0);
+                     int flags = 0,
+                     const wxIcon& icon = wxNullIcon);
 #endif // wxUSE_TASKBARICON_BALLOONS
 
 protected: