Ensure all calls to pkexec use g_child_watch_add()

Otherwise the application asserts with new versions of GLib...
trivial: Sort the whitespace in gs-offline-updates.c
Fix the name of the offline update file

Yes, it was a typo, and yes it's very hard to change now.
Initial port of the PackageKit session interface to gnome-software

Until we implement all the methods, use the org.freedesktop.PackageKit2 name.