OSX: Implement TaskbarManager for Mac OS X

This implements count badge, progress bar, and icon overlay.
It uses the NSDockTile API which is available since OS X 10.5.
The code compiles and run on older system but without doing
anything.

backends/platform/sdl/macosx/macosx.h

@@ -35,6 +35,7 @@ public:
 
 	virtual Common::String getSystemLanguage() const;
 
+	virtual void init();
 	virtual void initBackend();
 	virtual void addSysArchivesToSearchSet(Common::SearchSet &s, int priority = 0);
 	virtual void setupIcon();