Add Reality Adlib Tracker (RAD) format support

[AliceLR]

TODO:

• Fix warnings
• Add RAD 1.x support.
• set_order
• get_order
• set_position
• get_position
• get_length
• Add test

[asiekierka]

XMP allegedly supports .RAD files, which would be good for ensuring we can handle for the oddball platforms without a proper C++ compiler.

[AliceLR]

XMP actually does not support RAD files. All support for RAD/OPL S3Ms/other Adlib formats was removed in 4.4.0 because it was apparently really bad. Using the official RAD player also ensures we have RAD 2.x support (which other players like AdPlug currently don't). I think the concern about C++ being required is valid though (dunno which platforms that would be, but).

[asiekierka]

I think the concern about C++ being required is valid though (dunno which platforms that would be, but).

None of the ones we handle now, but as we continue looking into the absurd...