Fix `editor.selectedContainer`

@winhamwr a comment of yours was removed from here, where you were concerned with `getNodes`. So, now we have a wrapper for it, `_getSelectedNodes()` and that wrapper has some simple tests.

I say simple because they are too simple. I commented on that in the new tests.

I think that for now, this is a bit of an improvement on the code, and a great stride forward in our understanding of the selection topic, its quirks and possible steps forward.

So I think that we should leave this PR as it is, perhaps with a few more simple tests for `_getSelectedNodes`, such tests that are so simple, that they will actually pass in all of the browsers.