Mutation observer should filter out empty child list mutations

[Reinmar]

1. Load: <p><strong>Buold</strong> xxx</p>.
2. Try to use a spellchecker to correct "Buold" to "Bold".

You get the following mutations:

The first one is the change of the <p>'s children. This is fine.

However, the second one is a change of the root's children and it contains the same list of children.

Issue encountered in: ckeditor/ckeditor5-typing#110.

[Reinmar]

I can see on the list of native mutations that there are only two which touch the root:

And if we look closer it turns out that one adds a text node while the other removes it:

And that the text node is empty.

So, my guess is that spellchecker behaves a bit like a IME. It does some operations on the DOM but those operations are handled by some generic algorithms like "remove text", "insert text". Once they get a range at a block boundary they leak out of that block.

[Reinmar]

I pushed a branch t/1031 and it fixes cases mentioned in ckeditor/ckeditor5-typing#110.