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

interface/wx/notifmsg.h

@@ -10,12 +10,36 @@
 
     This class allows to show the user a message non intrusively.
 
-    Currently it is implemented natively for Windows and GTK and uses
-    (non-modal) dialogs for the display of the notifications under the other
-    platforms.
+    Currently it is implemented natively for Windows, OS X, GTK and uses
+    generic toast notifications under the other platforms. It's not recommended
+    but @c wxGenericNotificationMessage can be used instead of the native ones.
+    This might make sense if your application requires features not available in
+    the native implementation.
 
     Notice that this class is not a window and so doesn't derive from wxWindow.
 
+    @section platform_notes Platform Notes
+
+    @par OS X
+    The OS X implementation uses Notification Center to display native notifications.
+    In order to use actions your notifications must use the alert style. This can
+    be enabled by the user in system settings or by setting the
+    @c NSUserNotificationAlertStyle value in Info.plist to @c alert. Please note
+    that the user always has the option to change the notification style.
+    
+    
+    @beginEventEmissionTable{wxCommandEvent}
+    @event{EVT_NOTIFICATION_MESSAGE_CLICK(id, func)}
+           Process a @c EVT_NOTIFICATION_MESSAGE_CLICK event, when a notification
+           is clicked.
+    @event{wxEVT_NOTIFICATION_MESSAGE_DISMISSED(id, func)}
+           Process a @c wxEVT_NOTIFICATION_MESSAGE_DISMISSED event, when a notification
+           is dismissed by the user or times out.
+    @event{wxEVT_NOTIFICATION_MESSAGE_ACTION(id, func)}
+           Process a @c wxEVT_NOTIFICATION_MESSAGE_ACTION event, when the user
+           selects on of the actions added by AddAction()
+    @endEventTable
+    
     @since 2.9.0
     @library{wxadv}
     @category{misc}
@@ -53,6 +77,17 @@ public:
      */
     virtual ~wxNotificationMessage();
 
+    /**
+       Add an action to the notification. If supported by the implementation
+       this are usually buttons in the notification selectable by the user.
+
+       @return @false if the current implementation or OS version
+       does not support actions in notifications.
+       
+       @since 3.1.0	   
+    */
+    bool AddAction(wxWindowID actionid, const wxString &label = wxString());
+    
     /**
         Hides the notification.
 
@@ -69,9 +104,22 @@ public:
         Valid values are @c wxICON_INFORMATION, @c wxICON_WARNING and
         @c wxICON_ERROR (notice that @c wxICON_QUESTION is not allowed here).
         Some implementations of this class may not support the icons.
+        
+        @see SetIcon()
     */
     void SetFlags(int flags);
 
+    /**
+        Specify a custom icon to be displayed in the notification.
+         
+        Some implementations of this class may not support custom icons.
+        
+        @see SetFlags()
+        
+        @since 3.1.0
+    */
+    void SetIcon(const wxIcon& icon);
+    
     /**
         Set the main text of the notification.