xmms-mp4 (plugin to support .mp4/.m4a/.aac files in XMMS 1)

This adds playback support for mp4/m4a/aac files to XMMS 1. This is
for XMMS 1.x (the one shipped with Slackware). It's not needed for
xmms2 from SlackBuilds.org.

Seeking is not fully supported. It seems to work on some files at
least.
