Storing state in App

[daddycocoaman]

This discussion references #4107. It was labeled as answered by the topic creator, but I think it fell short a little bit.

I think it might be useful to have a context dict class for the App (similar to how Typer/Click have one) with a function to request the type of the context item as well. For example:

@overloads
get(name, type, default)  - function that also accepts a type, and returns that type. Returns a default if not found.
get(name, type) - function that also accepts a type, and returns that type. Raises an exception if not found.

Currently, you can add attributes to the App class you create, but accessing those attributes with type info from a Widget doesn't work because self.app returns an App[object] and not the created App class.

[and-viceversa]

Agreed on this idea and the linked discussion. I settled on a similar simple solution, but also had the impluse to reach for a click like Context feature. This seems to work for now but it seems like there should be a better way.

class MyApp(App):
    def __init__(self) -> None:
        self.context = {"state1": "", "state2": ""}
        super().__init__()