Initial pull of 5x5 into the Textual examples directory

[davep]

I think this is in a good enough state to start to pull it in now. It has keyboard navigation, mouse navigation, a simple help screen, a simple win state display, etc. It demonstrates not using a default screen, etc. Colour needs some work, but I think the core implementation is pretty sound now.

Tweaks to make it more Textually idiomatic are welcome -- this is obviously written by someone who is still getting to know the framework.

[willmcgugan]

Great! Just a few requests.

Once that's in, I might have a go at the stylesheet.

[darrenburns]

Could you change your import of Final to something like this?

import sys
if sys.version_info >= (3, 8):
    from typing import Final
else:
    from typing_extensions import Final