search: fix moving unsorted applications

Search providers can either have an explicitly configured sort order,
or be in the default alphabetical order based on their name.
Previously, when moving we would always assume that all providers had
a sort order, so moving an unsorted provider would put at the top.
The right thing to do, instead, is to walk the list in visual order
and assign an appropriate sort order, that is then flushed to gsettings.

https://bugzilla.gnome.org/show_bug.cgi?id=694975