Remove multimedia subdirectory

Unless it was a group of sub-packages, packages was never regrouped by
category. multimedia/ was an exception to this rule.

This patch move packages/multimedia/ sub-directories to packages/. It
keeps two subdirectories for gstream 0.10 and gstreamer 1.X.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>