Separate ffmpeg/mpv and audiomixer conditionals

[zoltanvb]

Description

There are cases when mixer exists without ffmpeg. In such cases, some music content can still be added to the mixer for playback. Logic added to separate the two cases.

Now it behaves as follows:

• music file extensions supported by either the mixer or the media player will appear in content list if "use builtin mediaplayer" is enabled
• "Add to mixer" / "Add to mixer and play" entries will be present if there is audiomixer and the respective format support is compiled in
• "Play in Media Player" (renamed from "Run") will be present if the media player supports the file. (Note: it needed an extra check because tracker modules like s3m are not supported by ffmpeg, and for wav, FFMPEG support for wav files is broken #3388 is still open so it will be horrible,)

There are a few minor improvements that could still be done: if there is any core that supports the particular music file type (such as ep128emu and wav support), "Run" will not be shown, unless "use builtin mediaplayer" is turned off, and "add to mixer and play" does not turn on mixer, although that would be convenient.

Related Issues

Closes #8451