[packagekit] GetUpdateDetail new API

[Richard Hughes]

I've just merged in some more API so that backends can optionally
return details about the update to the user. This lets people click on
the update viewer and get something sensible like "Fixes segfault in
the gbackend" rather than gibberish.

I've added stubs into the backends, and only have to connect up some
client side stuff, but I would appreciate it if you guys can figure
out if you can get update detail, even if it's only a URL or even the
package it replaces.

Yell on IRC or on the mailing list if you have questions or queries. Thanks.

Richard.