Accessibility mode

The UI code is separated from the logic for displaying pages, and it has an alternative text-based frontend already. Let's build this into a full accessibility mode.

From https://gitlab.com/postmarketOS/pmaports/-/issues/2321:

Accessibility functions that could be very useful for now is: screen reader (enable/disable by holding volume up and down in for 3 seconds, like on Androoid) maby a dark mode enabled by enabling screen reader and then volume up or down + power button for 3 seconds..

Maybe combine it with numen as done here: https://lists.sr.ht/~mil/sxmo-devel/%3C20221108215538.rhg6ja4m6nfv62s3%40trantor%3E