Error should be produced for callback with multiple aliases

[ogoffart]

Discussed in https://github.com/slint-ui/slint/discussions/3947#discussioncomment-7597955

export component Foo {
    callback clickedA <=> button.clicked;
    callback clickedB <=> button.clicked;
    button := Button {
        text: "OK!";
    }
}

Multiple callback to the same callback won't work and only one will be run

SlintPad

This should produce an error.

Also happens with Globals.