add ScreenType for screen's type identification

src/lastfm.h

@@ -40,6 +40,7 @@ struct Lastfm: Screen<NC::Scrollpad>, Tabbable
 	virtual void resize() OVERRIDE;
 	
 	virtual std::wstring title() OVERRIDE;
+	virtual ScreenType type() OVERRIDE { return ScreenType::Lastfm; }
 	
 	virtual void update() OVERRIDE;