Make fps of the visualizer configurable and set it to 60 by default

src/settings.h

@@ -82,6 +82,7 @@ struct Configuration
 	std::string lastfm_preferred_language;
 	std::wstring progressbar;
 	std::wstring visualizer_chars;
+	size_t visualizer_fps;
 	bool visualizer_autoscale;
 	bool visualizer_spectrum_smooth_look;
 	uint32_t visualizer_spectrum_dft_size;