Replace libmodplug with libopenmpt

[fdelapena]

There are several reasons:

• libmodplug is almost unmaintained and there are some security issues in module loaders sometimes.
• libmodplug does not support mo3 format (mod/s3m/xm/it with compressed samples).
• libmodplug has a grey grade license (plain public domain, not recognized on multiple countries).
• openmpt is widely supported in popular distros nowadays (including Debian, CentOS, openSUSE, Slackware, FreeBSD).
• openmpt gets frequent updates (~montly), currently 2 maintainers.
• openmpt FAQ contains further differences compared to libmodplug.

Compared to current libxmp, 2019 has 3 commits as the most recent ones.

If I recall correctly, current module player implementation in Player (not sure if libmodplug or libxmp) does not handle properly loopstart module commands, not matching @elvissteinjr's Audieremony patch behavior.

The openmpt API differs, so it is not the same API. We added xmp (#967) time ago because libopenmpt was not popular enough in 2016, but that's not an issue anymore, it's currently more accurate than libxmp and plays better. It could even replace libxmp by default.

[carstene1ns]

libopenmpt should be easy to integrate, we do not use libmodplug directly, so this is only about libxmp.
Back then another reason for the choice of xmp was the availability of the lite version for smaller binary size.
A static xmp-lite is 144K stripped, libopenmpt (shared, without dependencies!) is already 1,7M.

AFAIK the loopstart stuff is only for ogg files, so a separate (though easy) issue.