Fix set_default_icon_list() ownership transfer bug

Ownership of the GList is transferred in this call, so a copy should
be passed instead.  This will be fixed in a future version of Vala.

Upcoming versions of Vala will also not allow private members in
structs, so that is removed here as well.

https://mail.gnome.org/archives/vala-list/2014-August/msg00022.html