Serialize selections state

[domoritz]

In many applications people want to store the state of an application for example in the URL. Mosaic should have a way to read/write selections and all selections via the coordinator. To make the state stable, a selections could have an id (which probably should be set automatically if not specified).

[jheer]

This should be relatively straightforward for registered selections. However, if the ultimate goal is to share overall state, one will also want to export/import Param values, but these are not currently tracked by a coordinator.