Modify Message.control so that it is a @property and not a classvar

[davep]

The current implementation of the control property of Message results in the following type error (in this case taken from DirectoryTree.FileSelected.control:

Change this so that it is a @property on Message, like this:

def control(self) -> Widget | None:
    return None

and then modify the on decorator so that it does the pre-flight check of control by comparing the type's control with Message.control, raising an error if they are the same (not overridden).

[github-actions]

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