External modifications of the editing root should not crash the editor

[jodator]

Suggested merge commit message (convention)

External modifications of the editing root should not crash the editor. Closes ckeditor/ckeditor5#2016.

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Additional information

• I've found out that if we initialize the editing area directly as a Vue component root then Vue updates the children of the editing area on removing all nodes.
• This touches only EditorInline and BallonEditor as those are used directly as a component root. The ClassicEditor is not touched by this as its editing area is wrapped in another <div> which is then used as component root.
• More links here: Exception on delete value ckeditor5#2016 (comment).
• This behavior in Vue is didctated by some old Chrome bug (v. 55): textNode removed unexpectedly due to Chrome 55 bug vuejs/vue#6601

[jodator]

As it turned out the issue was already fixed by: ckeditor/ckeditor5-vue#52.