I/6116

[Reinmar]

Suggested merge commit message (convention)

Other: Introduced support for multi-range selections. Closes ckeditor/ckeditor5#6116. Closes ckeditor/ckeditor5#2010.

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Additional information

Accompanied with:

• I/6116 ckeditor5-typing#227
• I/6116 ckeditor5-table#267

By far the most tricky part of this task. I had to find the right balance between the deleteContent() (and hence insertContent()) doing reasonable things and overcomplicating them on the cost of maintenance issues and affecting some other cases (mostly: track changes).

By the occam's razor's call I chose the simplest solution – removing code. I removed some quite unnecessary behaviour from deleteContent() and hence was able to use the selection's real position in insertContent().

During the course of action I also stumbled upon a case where the schema's method was leaking out of the limit element and putting selection on a table cell instead. It was visible when pasting a word over multi-cell selection in an empty table. I fixed this too.

[Reinmar]

Those changes were reviewed already by @scofalik and @oleq. Merging.

[coveralls]

Coverage increased (+0.3%) to 100.0% when pulling b67cd98 on i/6116 into 968b193 on master.