Initial implementation

[pomek]

Suggested merge commit message (convention)

Feature: Initial implementation. Closes ckeditor/ckeditor5#1194.

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Additional information

We need to improve the demo but it can be done outside this PR.

[pomek]

I improved the demo snippet. I added a button that opens a print preview window after clicking:

For the above content, it will show something like on the picture below:

[pomek]

@dkonopka, could you check whether the pageBreak widget is being rendered correctly with float images?

[dkonopka]

WFM 👍

[mlewand]

Overall looks fine.

I don't know why data is downcasted the way it is thoug, see the review comments for more details.

@MSGY I'd like to ask you to give it a quick testing. It is very similar to the horizontal rule (line) feature, but I'd still love to have another pair of eyes to check it.

@pomek Please make a 0.0.1 pre-release so that CI already kicks in.

[mlewand]

Looks fine now.

While doing 47135ef you didn't adjust the TC in https://github.com/ckeditor/ckeditor5-page-break/blob/a8b6d7b/tests/pagebreakui.js#L47 which caused tests to fail. I'll fix that as a part of R.

I also added a reference to your comment explaining putting span inside a div.