Unify widget insertion

[jodator]

Suggested merge commit message (convention)

Other: Table feature should insert table the same way as other widgets. Closes ckeditor/ckeditor5#3181.

BREAKING CHANGE: The TableUtils#createTable() method now accepts model Writer instance instead of Position. The method no longer inserts created table to the model - use returned value instead.

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Additional information

• Part of: T/ckeditor5/1243: Unify widget insertion ckeditor5-engine#1545

[coveralls]

Coverage decreased (-0.5%) to 99.52% when pulling 974478f on t/27 into 0a53042 on master.