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

samples/dialogs/dialogs.h

@@ -485,9 +485,7 @@ public:
 
     void OnRequestUserAttention(wxCommandEvent& event);
 #if wxUSE_NOTIFICATION_MESSAGE
-    void OnNotifMsgAuto(wxCommandEvent& event);
-    void OnNotifMsgShow(wxCommandEvent& event);
-    void OnNotifMsgHide(wxCommandEvent& event);
+    void OnNotifMsg(wxCommandEvent& event);
 #endif // wxUSE_NOTIFICATION_MESSAGE
 
 #if wxUSE_RICHTOOLTIP
@@ -517,10 +515,6 @@ private:
                         *m_dlgReplace;
 #endif // wxUSE_FINDREPLDLG
 
-#if wxUSE_NOTIFICATION_MESSAGE
-    wxNotificationMessage *m_notifMsg;
-#endif // wxUSE_NOTIFICATION_MESSAGE
-
     wxColourData m_clrData;
 
     // just a window which we use to show the effect of font/colours selection
@@ -612,9 +606,7 @@ enum
     DIALOGS_FIND,
     DIALOGS_REPLACE,
     DIALOGS_REQUEST,
-    DIALOGS_NOTIFY_AUTO,
-    DIALOGS_NOTIFY_SHOW,
-    DIALOGS_NOTIFY_HIDE,
+    DIALOGS_NOTIFY_MSG,
     DIALOGS_RICHTIP_DIALOG,
     DIALOGS_PROPERTY_SHEET,
     DIALOGS_PROPERTY_SHEET_TOOLBOOK,