Remove InteractiveSearchBox entirely

[QuLogic]

This widget was added in d426f6232e2069de9f051ef5b0f45513bfa55b9c as a copy of the original GtkTreeView code, with a modified timeout to the start of searching. Since then, there has been 1 additional change to functionality in bb26e53fc0cb3989c05ee302052d635a61ec6b3d to add support for a slightly different version of PersistentTreeView on ListView. All other changes have been minimal maintenance: fixing small bugs, replacing deprecated GTK calls, or code formatting.

I contend that this is too much code to maintain for such a small change in functionality from upstream GTK, and any migration to new widgets in GTK4 will have to implement this differently anyway, so I'm proposing to remove it all. AFAICT, search still works without this code, just with GTK's timing instead.