Don't trigger listener for not updated state listener changes

[christian-bromann]

If you listen to a certain state in tangle, e.g.:

interface State {
    propA: boolean
    propB: string
}

const ch = new Channel<State>('foobar', {
    propA: true,
    propB: "foobar"
});
const bus = await ch.registerPromise([ ... ])
bus.listen('propB', (val: string) => { ... })

The listener is also triggered even if the property wasn't changed, e.g.:

client.broadcast({ propA: false })

This causes unnecessary listener calls and could potentially decrease performance. Let's ensure to only trigger the listener if the property was changed.