COSDFader: rename member functions, adapt implementations

Stop() is not exactly attributable if fader class used as inherited and
Fade() don't tell really what done and related to return value 'bool'...
especially as the fader class members have been not documented.

src/driver/fade.h

@@ -36,12 +36,12 @@ class COSDFader
 		CFrameBuffer            *frameBuffer;
 	public:
 		COSDFader(unsigned char &alpha);
-		~COSDFader();
+		virtual ~COSDFader();
 
 		void StartFadeIn();
 		bool StartFadeOut();
-		void Stop();
-		bool Fade();
-		uint32_t GetTimer() { return fadeTimer; };
+		void StopFade();
+		bool FadeDone();
+		uint32_t GetFadeTimer() { return fadeTimer; };
 };
 #endif