Have GeglVisitor::visit_{pad,node}() return gboolean, instead of
void.  Returning TRUE from these functions terminates traversal
early.  This is useful, e.g., when searching a specific node in a
graph.

Have gegl_visitor_{dfs,bfs}_traverse() return a gboolean, instead
of void, indicating whether traversal was terminated early.  This
is useful, e.g., to determine whether the node was found.

Adapt the rest of the code to the changes.