Add song trim feature

[PKBeam]

We should add a feature that allows users to trim the audio file so it begins on a beat, or add silence at the start to achieve the same.

This can be done using the bundled ffmpeg binary (e.g. here). May have to rebuild ffmpeg with support for that concat demuxer?

Users should be able to specify how many beats of silence should precede the song beginning.

[Brollyy]

In the new in-game map editor, Wanadev included option to change song start offset. This seems to correspond to _songTimeOffset property in info.dat file. Maybe this can be used to align the song without having to do any audio file manipulation?

[Brollyy]

In the new in-game map editor, Wanadev included option to change song start offset. This seems to correspond to _songTimeOffset property in info.dat file. Maybe this can be used to align the song without having to do any audio file manipulation?

This property doesn't seem to do anything. I've tested changing it both manually and with the in-game map editor, and in both cases it seems to have no impact when playing the map. Amusingly, the only thing it does in the in-game editor is break the grid alignment for placing new notes.

So doing something with ffmpeg will be required for this... Unfortunately, I won't have that much time to implement this for the time being. Putting this back into backlog.