Remove gegl_visitor_{dfs,bfs}_traverse().  They weren't actually
performing DFS and BFS, but were rather characterized by performing
topological (dependencies-first) and reverse-topological
(dependencies-last) search.

Instead, add gegl_visitor_traverse_[reverse_]topological(), which
perform a forward- and reverse-topological search, with lower
overhead (in the BFS case, the specific order is different than
before.)

Adapt the rest of the code to use the new functions.  In
particular, use a single GQueue in GeglGraphTraversal to store the
path in topological order, instead of separate DFS and BFS paths.
We forward-iterate over the path where we'd previously used the DFS
list, and reverse-iterate where we'd previously used the BFS list.

Remove GeglListVisitor, since it's no longer used.