Disconnect FlattenedNodesObserver on disconnectedCallback

[mperktold]

The FlattenedNodesObserver created in the constructor is never disconnected:

collapsible-splitlayout/src/main/resources/META-INF/resources/frontend/src/collapsible-vaadin-split-layout.js

Line 290 in 98bf450

new FlattenedNodesObserver(this, this._processChildren);

The docs suggest that this should be done:
https://polymer-library.polymer-project.org/3.0/api/utils/flattened-nodes-observer

Otherwise, I suspect this leads to memory leaks. In our application, we saw several objects retained by detached DOM nodes, and for many of them, this component shows up in their retainers. That's of course not a proof that this component is the root cause, but since there is this thing going on with the FlattenedNodesObserver, I can't rule it out either.