Allow more fine-grained control of audio

[carstene1ns]

I guess adding some volume control is currently not needed (Scene_Options anyone? :grinning:), but I think it would be a good idea to allow disabling of music and sound effects separately. For example I like playing games with only sound effects and having my own music in background.

[fdelapena]

Indeed, some MV-like feature for this would be really useful, also until broken MP3 playback is being fixed :laughing: .

[Ghabry]

Combined with #666 the setting of the volume control could be stored globally.

All this stuff could be placed in the "Settings" dialog that was added to RPG2k3 English (and F-special key binding and Android ingame menu entry). The real RPG2k3 shows some Windows dialog with stuff, but we can do better :P