lumino: preserve layouts in LocalStorage

[oliver-sanders]

Currently the UI will remember tab layouts within a session but not between them.

Using LocalStorage would allow tab layouts to be preserved between sessions. This would be especially relevant for saving the default layout (#1834) and restarts of the UIS.

Preserving layouts presents the additional complexity of working out how to behave when multiple sessions are present.

See #662

[MetRonnie]

Need to investigate how to JSON serialise the lumino widget objects