Simple Mode - Ability to toggle between simple and advanced admin features

Description of the need

@cellear described a feature idea in the dev meeting today that sounded interesting. He suggested that a site owner should be able to simplify the admin menu WITHOUT having to change roles. New site owners can often be distracted by the many configuration options in the admin menu that do not apply to what they need to be doing. One solution is to login with a less permissive role.

Proposed solution

@cellear suggested adding the ability for a site admin with broad administrative permissions to have the option of turning off the advanced admin features, while maintaining access to the simpler ones. Maybe turn on "simple mode" without having to logout and with the ability to turn on the advanced features at any time.

Alternatives that have been considered

We added the editor role to Backdrop CMS out of the box to address this use case. However, this doesn't fully address the use case outlined here.

This might be a good candidate for a contrib module first?