Upcast writing and View enhancements

[f1ames]

Suggested merge commit message (convention)

Other: Introduced createDocumentFragment(), createElement() and createText() methods in UpcastWriter. The View.change() method now returns result of its callback. Closes ckeditor/ckeditor5#4423.

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Additional information

See ckeditor/ckeditor5#4423 for details.

[f1ames]

There is related PR (change in docs) in clipboard plugin - ckeditor/ckeditor5-clipboard#59.

[Reinmar]

As in comments.

[Reinmar]

A control question – did you check for new Element/DF/Text() use in at least API docs? I'm sure there's a lot of that in tests, but that we can leave (it's also correct cause constructors are protected, not private).

[coveralls]

Coverage remained the same at 100.0% when pulling b0aa93b on t/1549 into 18bab70 on master.

[f1ames]

When it comes to b0aa93b, I have changed all API samples which used protected constructors (e.g. new Element()) to use DowncastWriter.

It could be also changed to use UpcastWriter, however as UpcastWriter doesn't provide methods to create specialized elements (like downcastWriter.createAttributeElement()) some samples would require using both downcast and upcast writer which may look confusing.