lumino: allow saving named layouts

[oliver-sanders]

Users may use different tab layouts for different applications.

It might be a useful feature to allow the current layout to be saved as a named layout which can be restored in any workspace at a later date. Nice to have.

This is point (4) from #662

Named layouts (nice-to-have).

Allow users to save "named" layouts.
Allow users to re-configure the workspace to a previously saved layout.