Modify MapMenu to maintain map state

[jbirch]

Currently the experience for users switching between maps in the MapMenu is non-intuitive. When they switch from one map to another and then back again, they lose their layer state, selection, etc from the first map. Numerous users have complained about this defect in our Theme picker.

It would be extremely useful if the MapMenu widget could be modified so that when a map in the menu is loaded, it modifies the MapMenu widget on the fly to reference the runtime version of itself in the current session rather than a map from the library, and so that when re-loading it just uses itself rather than creating a new runtime map.