setup: Adjust positioning of sound dialog.

When configuring Chocolate Strife the sound dialog can become quite
tall, and the dialog is visibly uncentered vertically. Move it up
slightly to counterbalance it.
setup: Add missing argument to M_StringJoin.

Last argument to M_StringJoin needs to be NULL.
music: First code for HQ music substitution.

This adds support for high quality music packs that replace Doom's
built-in MIDI music with digital recordings. In particular this allows
recordings of the Roland SC-55 to be used in Chocolate Doom. This is
the first essential step for bug #245.
music: Minor tweaks to substitute music code.

Use appropriate directory separator for OS when generating config file
paths. When generating config files, include a header line that
indicates what is being assigned.
Fix typo in README.Music.

Thanks Gaerzi.
music: Suggest using .ogg format, not .mp3.

Using the .mp3 extension in generated music config files carries an
implicit recommendation to use that format. There are a number of
reasons to prefer other formats. The biggest is that MP3 is patent
encumbered. Furthermore, while FLAC and Ogg Vorbis both support the
Vorbis comment format that allows metadata loop point tags to be
specified.
Merge branch 'master' of https://github.com/chocolate-doom/chocolate-doom