Observables: search inside added/removed nodes

[horacio-penya]

We're working with pages that are rendered depending on API requests, so we do something like this:

  {
    selector: '#create_question',
    content: 'Click on + Query',
    highlightedSelectors: ['#create_question'],
    mutationObservables: ['#create_question'],
    resizeObservables: ['#create_question'],
  },

Because of some interaction between react-dom and Intersection Observer API that I don't really understand, mutationObservables is not deterministic, sometimes it gets the element I'm looking for (<div id="create_question">...</div>), sometimes it gets an ancestor (<div ...>...<div id="create_question">...</div>...</div>) and not always the same ancestor. When it gets an ancestor, it doesn't match the selector and so it doesn't refresh.

This PR makes Observables search inside the added/removed elements instead of just looking at them.