* src/filetree.{cc,h} (FileTree::next_match_file): Replace the
collapse_stack parameter by a single Gtk::TreeModel::Path pointer,
because we really don't have to collapse each expanded child row
explicitely.  Simplify the implementation accordingly.
(FileTree::prev_match_file): ditto.
(FileTree::select_next_file): Get rid of the collapse_stack
container and use a simple Gtk::TreeModel::Path variable instead.