Decorator approach to event handling

[willmcgugan]

Add a decorator approach to handling messages, in addition to the naming convention.

Implement a function called on, importable from the root. This would take the message type, and an optional selector.

The message system would check for decorated handlers before the naming convention.

Note: there could be more than one handler associated with a method.

from textual import on

class MyApp:

    @on(Button.Pressed, "#quit")
    def quit_app(self, event:Button.Pressed):
        ...

I think Textual should look for the decorated handler first, then the naming convention.

Note that we have a sender attribute, which for the most part could be used to match against the selector. Consequently I think we need a method on the message itself, so that any given message hand override the matching behaviour.

[willmcgugan]

Updated to use @on which compliments the current naming convention, and reads better.

[willmcgugan]

#2453

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