Don't allow screen instances in App.SCREENS

[willmcgugan]

Textual currently allows a screen instance to be specified in the SCREENS classvar. This works fine when launching an app, until a second instance of the app runs. Re-using a previous instance breaks the app.

See the example here: https://textual.textualize.io/guide/screens/#creating-a-screen

Note how the BSOD is given as an instance in a classvar, i.e. SCREENS = {"bsod": BSOD()} This would cause the instance to be shared if there is ever more than a single App.

The easiest workaround is to disallow passing instances, and require a string or callable. So the above example should be SCREENS = {"bsod": BSOD}

Requirements:

• Adjust typing to not allow a screen instance in SCREENS and MODES
• Reject screen instances at runtime, by throwing a TypeError.
• Update docs and examples accordingly.
• Tests to ensure behavior when not using a string or callable.

[ZeroIntensity]

I can give this a go!

[willmcgugan]

Good catch re modifying the MODES classvar. We should not be doing that. Suggest we copy the classvar to an instance var.

If that looks straightforward. You could do it in this PR. Otherwise, it could be a new issue.

[ZeroIntensity]

Sounds easy enough, I'll implement it in this PR.

[ZeroIntensity]

Added it under a new modes attribute!

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