Download updates

This moves more functionality from the gsd updates plugin
here:

* Notify about successful or unsuccessful offline updates
  15 seconds after start.
* Monitor for available offline updates and notify about
  them, once per hour.
* Refresh the cache once per day, after 6am.
* If important updates are available, or updates have not
  been installed for at least 7 days, download all
  available updates. We rely on PackageKit to prepare
  an offline update when updates have been downloaded.
* Avoid mobile networks for cache refresh and downloads.

https://bugzilla.gnome.org/show_bug.cgi?id=709121
Use proxy settings

This moves more functionality from the gsd updates plugin.

We pass the session proxy settings to PackageKit, so it can
use the right network for downloading.