* packagekit: disable nix-backend
Packagekit fails to build on my machines, as long as it's nix-backend is enabled
* packagekit: add 'enableNixBackend' as an option
- currently pulled in from Git until the next release of PackageKit
has Nix support
- also: add in a service module to start packagekit properly
- nixos service can be enabled via services.packagekit.enable
- packagekit requires nixunstable to build properly
This will probably be mandatory soon, and is a step in the right
direction. Removes the deprecated meta.version, and move some meta
sections to the end of the file where I should have put them in
the first place.