gtkfilechooser: Keep GtkSearchEngine around for longer

gtkfilechooser: Keep GtkSearchEngine around for longer

Currently the GtkSearchEngine is torn down every time the search
is stopped, which also means between typed characters. This
prevents any of the optimizations that the GtkSearchEngine can
do in the long run.

Let the GtkSearchEngine stay around for longer, and only be
disposed after search is cancelled, the filechooser moves
onto a different mode than search, or is otherwise unmapped/disposed.

While at it, remove an unused struct field.