search: fix description of results

"description" is documented as a valid field for search result metas,
and ListSearchResults implements it, so pass it down to be used.
Also, don't wrap the description in quotes, so that the search provider
can decide if it is an excerpt from the searched text or something else.
And to that extent, set use_markup to true, so that terms can be
highlighted.

https://bugzilla.gnome.org/show_bug.cgi?id=694906