multi-file-mode switch interaction

[mwestphal]

Is your feature request related to a problem? Please describe. With the addition of a grouping logic (--multi-file-mode) in https://github.com/f3d-app/f3d/pull/1609, it would be nice to be able to change group dynamically

An interaction could be added to switch multi-file-mode

Probably require https://github.com/f3d-app/f3d/issues/443 first