This rejiggers a bit the way selection focus is handled so that focusing a
disabled form control with the mouse handles selection properly, and hides the
document selection and so on.
This matches the behavior of other browsers as far as I can tell.
Given now readonly and disabled editors behave the same, we can simplify a bit
the surrounding editor code.
Differential Revision: https://phabricator.services.mozilla.com/D66464
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extra : moz-landing-system : lando
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