T/629 Fix infinite selection change loop

[scofalik]

Fixes ckeditor/ckeditor5#3843.

The problem with selection was that during conversion chain: DOM -> view (1) -> model -> view (2) -> DOM view (1) selection was different than view (2) selection because there was a difference how model -> view and DOM -> view selection is converted. This caused an infinite loop of selection fixing.

I changed one of conditions that was responsible for breaking the loop. The condition looked whether view (1) and view (2) are equal. Now I check whether they are touching. If they are, the selection is not converted any further.

To do so, I needed to implement isTouching in model range, selection and view position, range selection.

[scofalik]

I provided an alternative solution in #671

[Reinmar]

Since we're betting more on #671 I'm closing this.