window: reimplement search

We previously had a dedicate view for handling search,
based on model filtering and a custom panel to display
that differently.

After moving to GtkListBox, search can be trivially
done by using a filtering function, and widgets can
be fine-tuned to display extra information.

This patch, then, reimplements the search using a filtering
function over the panels' list.

https://bugzilla.gnome.org/show_bug.cgi?id=766922