Fixed view.Renderer bug causing editor crash in some scenarios involving reusing DOM elements

[scofalik]

Suggested merge commit message (convention)

Fix: Fixed view.Renderer bug causing editor crash in some scenarios involving reusing DOM elements. Closes ckeditor/ckeditor5#6092.

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Additional information

To fix the issue, I inverted the order in which the actions in _updateChildren() happen. Earlier, after getting a diff, actions were performed one after another, as in diff array. I've changed it, so now deletions are performed first and insertions later. This prevents the situation where we are inserting a child which is still in that parent (as described in the ticket).

There was an alternative solution, where instead I did Array.from() on actualDomChildren and then was updating actualDomChildren in the diff loop and then when delete action was handled, I was checking if the element was already in actualDomChildren.

I think that the solution from PR is safer, though.

I've added only one unit test to confirm that the solution is working. The test is a scenario described in ckeditor/ckeditor5#6367. It tests exactly the described problem, that is, moving a DOM element "back".

The umbrella issue mentions a couple of issues.

I've tested:

• There should be no errors when indenting a code block  ckeditor5#5910
• Editor crashes after pasting the content multiple times ckeditor5#1579
• Renderer#_updateChildren fails when DOM element is "moved back" ckeditor5#6367

I haven't tested:

• Occasionally error when Move range between different roots. ckeditor5#5601
• When there is a lot of content in root,paste other content which's first element is a picture. Error. ckeditor5#5843

TODO:

1. Please confirm that the fix works. It would be a good idea to try to test the cases I haven't checked.
2. Write more unit tests that would focus on the cases described in those issues.

[coveralls]

Coverage remained the same at 100.0% when pulling 91f95ba on i/6092 into 3564ce5 on master.