PackageKit integration

[Richard Hughes <hughsient gmail com>]

gnome-color-manager in git master now integrates with PackageKit in
the following ways:

* Offers to install argyllcms if the user tries to calibrate without
argyllcms installed
* Offers to install shared-color-targets if they are not installed,
and the user tries to calibrate a non-display device

This means gnome-color-manager can be shipped on live-cd's without
bringing in the 5.0Mb argyllcms, and the 1.4Mb (and rising)
shared-color-targets, just in case the user wants to calibrate a
device.

If people want to turn off the PackageKit bits, then I can add a GConf
key or #ifdef as required.

Richard.