[packagekit] New role available to backends: UpgradeSystem

[Richard Hughes]

In a move that probably surprises nobody, I've created a backend
virtual UpgradeSystem(string distro_id) method. This allows frontends
to one day do the distribution upgrade (e.g. from Fedora 13 to Fedora
14) in a GUI tool, without running a toolkit as root, and perhaps even
do the distro upgrade semi-automatically in the future.

I've fixed up all the backends, so if you're not intending to use this
functionality then you can ignore this email.

In related news, we'll soon be porting the Fedora specific preupgrade
to the new GetDistroUpgrades() and UpgradeSystem() PackageKit
framework. It might make sense to abstract some of that code for other
distros too.

Richard.