Add KSS input plugin based on libkss

Supports all MSX sound chips: PSG, SCC, OPLL/FMPAC, MSX-Audio/OPL.
Per-channel stereo panning for PSG and SCC, OPLL channel stereo.
Parses NEZplug extended m3u/m3u8 playlists for track titles, durations,
and fade times. Configurable via Audacious preferences panel.

libkss (ramonsmits/libkss per-channel-pan branch) included as submodule.

Author: ramonsmits

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "src/kss-libkss/libkss"]
+	path = src/kss-libkss/libkss
+	url = https://github.com/ramonsmits/libkss.git
+	branch = per-channel-pan

meson_options.txt

@@ -34,6 +34,8 @@ option('cdaudio-cddb', type: 'boolean', value: true,
        description: 'Whether CDDB support for the Audio CD plugin is enabled')
 option('console', type: 'boolean', value: true,
        description: 'Whether Game Music Emu support is enabled')
+option('kss-libkss', type: 'boolean', value: true,
+       description: 'Whether KSS (MSX music) support via libkss is enabled')
 option('ffaudio', type: 'boolean', value: true,
        description: 'Whether FFmpeg support is enabled')
 option('flac', type: 'boolean', value: true,