Updates to ubuntu-desktop and desktop-extra package sets

[Jeremy Bicha]

Please add gnome-icon-theme-symbolic to desktop-extra. It is listed at
http://git.gnome.org/browse/jhbuild/plain/modulesets/gnome-suites-core-3.4.modules

Also, please move yelp-xsl and yelp-tools from core to ubuntu-desktop
as those are GNOME core dependencies.

I'd also like to see ubuntu-docs moved to ubuntu-desktop, unless we'd
be better off making a separate package set for ubuntu-core-doc stuff.

Thanks,
Jeremy Bicha