search-provider: Use panel IDs as results instead of tree model paths

Since we now re-sort the model according to the search terms, we can't
use tree model paths as results since they're not stable. Instead
we'll use panel IDs and keep a map of IDs to GtkTreeIters to retrieve
the result metas from the model.

Caching GtkTreeIters in this way is OK because the model is a
GtkListStore which guarantees that while a row exists, the iter is
persistent.

https://bugzilla.gnome.org/show_bug.cgi?id=729027