Undo/Redo freezing ST when read_only is set

[bordaigorl]

ST would freeze (and has to be killed) during a coqtop session when
the cursor is positioned after the "proven" region, so the view is not read-only, but an undo action affects the contents of the proven region.
To workaround this issue I disabled undo/redo when the session is ongoing, but it's less than ideal,
see https://github.com/bordaigorl/Sublime-Coq/tree/fix-undo-readonly

Ideally, one would be able to predict the extents of the undo/redo action but that's not supported by the current API.
A less efficient (conservative) workaround would be to save the depth of the history-stack when executing "next statement", and disallow undo/redo beyond that depth.
I don't like any of these options.
Apparently there is no way to set read-only just for a region.