brasero.desktop.in.in: add Audio and Video categories

Otherwise Brasero won't show up in the GNOME software centre. See:
http://blogs.gnome.org/hughsie/2013/09/03/broken-desktop-files
Rely on intltool 0.50 native gsettings support

This lets us drop use of INTLTOOL_XML_NOMERGE_RULE in data/Makefile.am

https://bugzilla.gnome.org/show_bug.cgi?id=708750
Add community-produced appdata file

https://bugzilla.gnome.org/show_bug.cgi?id=708750