Switch back to upstream tcell

[jesseduffield]

Is your feature request related to a problem? Please describe.
We switched to @stefanhaller 's fork in order to get focus events working, but that PR has now been merged: gdamore/tcell#599

So I reckon we should switch back to the upstream.

BUT: I'm open to the alternative of instead continuing to use our fork in case we need some other bespoke feature that takes a while to get merged into the upstream.

[stefanhaller]

I had planned to switch back to upstream as soon as possible, but wanted to wait until there's a tag so that we don't have to reference a random head commit. Not sure this really matters, happy to switch now if you prefer.

On the other hand, we might also wait with switching back until something new is committed to upstream; until then there's little reason, really.

[jesseduffield]

The actual reason I was thinking about this was from seeing this new commit: stefanhaller/tcell@894250e

It allows you to lock cells so that they don't render, and I was interested to see if we could use that in #2966. But I suspect it wouldn't simplify things much.

So happy to wait until there's a good reason to switch back

[stefanhaller]

We have switched back to upstream two years ago (see #3194).