Use MutationObserver to setup widgets that are added after the initial load

[merlinfuchs]

Some customers had problems with widgets not loading correctly when they are dynamically added (e.g. in a modal). This is because we only call setup once on page load. Usually a customer would use our JavaScript library to setup the widgets themselves, but this doesn't really make sense for wordpress.
To solve this we could use a MutationObserver to listen for newly added HTML elements. I'm not sure about the performance if we are listening for DOM changes globally so there probably should be a way to turn it off in the settings.