Disregard property changed events for initial values

Lots of Vaadin components dispatch property change events even for initial values. That is due to how Polymer works, which triggers property changed events even if the property has not been set before. This has the effect that showing a large form in its initial state will already add a lot of event spans, even if the user did not interact with the form yet.

We should look into whether we can skip creating these event spans if they refer to setting the initial property value. That might be possible by comparing the value of the event with the value of Element.getProperty.

This should cover at least value-changed events, but we can also try to cover other events like opened-changed.