Blockquote implementation

[szymonkups]

This is an initial issue describing Blockquote feature implementation.
There is no requirements list yet, but we already see that it will require some changes in the engine. Let's gather them here.

Add support for nested elements to DataController#modifySelection:

Extending selection in situation like this:

<blockquote><p>foo[]</p></blockquote><p>bar</p>

should return:

<blockquote><p>foo[</p></blockquote><p>]bar</p>

[Reinmar]

I'm also curious how's deleteContent() dealing with such a cases. I remember that I didn't want to think too much on it back then so the implementation may be poor.

[Reinmar]

Besides, it's tricky to understand which rules are specific to blockquote and which are general and should land in the default algorithms.

[fredck]

Btw, be sure to check Editor Recommendations on Quotes.

[Reinmar]

Good point. Do we want to have a caption support? If so, then it's a far more complex feature than what we have in CKEditor 4. It may need to be implemented with a widget or some smart model operations. The latter would be a very interesting test for our data model, but I'm afraid that we don't have time for that now.

We'd need to start this with a UX research too, cause I'm not sure how should the caption be reachable when vs how to escape blockquote. Perhaps it's a simple rule that enter in an empty paragraph in a blockquote removes that paragraph and moves selection out of the quote and below the caption.

[Reinmar]

I talked with @fredck. We're going now with a CKEditor4 like implementation (but rendering to a <figure class="quote">). We'll rethink the captioning in the future.

[szymonkups]

When modifying selection in situation described in first comment will be implemented, it should also support objects. Widgets implemetation should also use it during delete and backspace handling:

<widget></widget><blockquote><p>[]</p></blockquote>
// backspace pressed
[<widget></widget>]

<blockquote><p>[]</p></blockquote><widget></widget>
// delete pressed
[<widget></widget>]

[Reinmar]

I can see that modifySelection() is covered by describe( 'beyond element – skipping incorrect positions' ) and works as expected.

So the problem with https://github.com/ckeditor/ckeditor5-block-quote/issues/7 lays inside deleteContent().