PackageKit: make use of the summary in details, when available

If PackageKit is recent enough, use the summary from GetDetails
as the application summary for normal apps, and use it as the
application name for local filenames (in place of the ugly
package name).

https://bugzilla.gnome.org/show_bug.cgi?id=724669
Remove Settings category from the desktop file

If we specify Settings togheter with System we end up in
the "Other" category in the default layout.

https://bugzilla.gnome.org/show_bug.cgi?id=723782