Handle "layers" permalink parameter

This PR adds support for handling "layers" in permalink. BOD layers only are handled for the moment!

The permalink is updated when layers are added, removed, or changed order in the map. And, at application loading time, layers specified with the "layers" parameter of the permalink are added to the map.

As [discussed](https://groups.google.com/d/msg/re3-dev/KNuTfnS0LcE/dNdP5x_cKHEJ) on the mailing list the permalink information supersedes the topic information. So, at application loading time, we ignore the selectedOpen flag of tree leaves if there are BOD layers specified in the permalink.

This PR builds on the foundation of #506, which needs to be merged first.