Feature: keepMounted prop for the Modal component to keep it mounted

[Rolv-Apneseth]

Is your feature request related to a problem? Please describe

Correct me if I'm misunderstanding, but currently, hiding a modal makes it lose all of its state because it gets unmounted.

Describe the solution you'd like

I would like to be able to hide a modal, without the modal losing all of it's associated state

Additional context

Same request as issue #2160 but that was closed