Bump Textual version, drop Python 3.7

This bumps the minimum Textual version to the most recent version, and also drops support for Python 3.7.

Also changes the way the package is uploaded to PyPI so that I can make it installable via Homebrew.

Upgrade Textual

Just upgrading Textual to the latest version.

Another playground

As well as some internal tweaks and small cosmetic changes, this adds another playground that concentrates on querying IDs and classes.

Adding multiple playgrounds

Adds support for multiple playgrounds. For the moment this just adds a second simple one with a sprinkle of widgets within it. Just trying the idea on for size.

Code tidying and UI tweaks

Mostly a bunch of code tidying and UI tweakage, cleaning up the evidence that this started life as a quick hack one evening. While it's not much more than a quick hack still, at least it's a tidy one now and can be built on.

Better isolate the playground

Because the playground was a Vertical, if someone where to query (for example) 'Vertical > Vertical' it would match the top container in the DOM, which would be surprising given how the playground presents itself.

This release makes a custom container and, while not making the name impossible to guess, it does isolate it from the usual CSS types that a user might try.

Initial prototype

The initial prototype of the idea.