[Proposal] Allow focus on disabled controls

# Problem

I always wondered, and was recently remembered by it again in https://github.com/fluentribbon/Fluent.Ribbon/issues/1121 by @cbra-caa, why focus on disabled controls is completely blocked.
IMHO this reduces accessibility as people with disabilities are completely blocked from accessing disabled controls using the keyboard.

# Proposal

I propose that we, optionally, allow focus on disabled controls.
That way application developers could add accessibility options to their applications to allow focus on disabled controls.

# Implementation

To implement this we could add a new property like `FocusableWhenNotEnabled` which would unblock focus on disabled controls.

I will give this a try and create a PR in the near future to explore which areas of WPF would have to be modified to make this possible.

If anyone has suggestions, concerns or any other kind of feedback regarding this i would be happy to hear from you.