[6.x] Fix non-localizable button group fields being editable in localizations

[lazerg]

A button_group field with localizable: false stayed clickable in a localization, and clicking an option desynced the field and wrote the new value into the localization's own content file instead of leaving it to resolve from the origin.

This used to work. fb4f58b swapped the fieldtype's :disabled="isReadOnly" for :read-only="isReadOnly", but Button was never given a readOnly prop, so since then the binding has landed on the element as an inert attribute and the click handler has stayed live.

Added the prop and folded it into the button's disabled state, the same way the Radio component handles it. The control appearance fieldtype and the revealer fieldtype's button mode pass the same prop to Button, so those are covered too.

Fixes#15149