Replaced "Zero Install" XDG submenu with a single normal menu entry

There's now an Add button in the list window, so that can be used for both, and
you can now also drag-and-drop directly to the list, which seems more natural.

Apart from being a bit cleaner, this should work better on systems which don't
have menus (e.g. GNOME 3, Unity).

Reverts db71bb19958.