Implement DataController.insertContent()

[Reinmar]

Needed for ckeditor/ckeditor5-clipboard#1.

It seems that it must accept view document fragment, because it will need to convert it to the model using the context of insertion. And this becomes part of the "insert content" logic, so should most likely be done inside this method.

BTW, it made me think that this method would make sense on the DataController. And I kinda like this idea. We've been thinking where to move the composer and I can see that modifySelection() makes for the EditingController and deleteContent() together with insertContent() could perhaps end in the the DataController. Note also that we'll need something like getSelectedContent() for the copy and cut operations (so deleteContent() would become deleteSelectedContent()).

OTOH, I know what you're going to say – those methods look weird on the DataController. I agree... I'd have to get used to them. But consider this – let's say we'd like to move insertContent() to the Document. But insertContent() requires (as explained above) a knowledge about DataController. So how could it be implemented on the Document?

tl;dr:

• DataController#insertContent() – paste and I'm sure it'll be used in many ways in many cases by developers,
• DataController#deleteSelectedContent() – delete feature,
• DataController#getSelectedContent() – copy, cut and this has been also requested many times in the past for various integration scenariosm
• EditingController#modifySelection()

Does it make any sense or no sense at all?

[Reinmar]

There's +1 from @pjasiun and @fredck for having these method on the DataController. Note, however, that they will have to accept the selection on which they operate, just like now, when they are in the composer. This is fine and gives the developer the ability to specify on what piece of content the operations should be made.

[Reinmar]

PS. modifySelection() makes most sense in the model.Document or on the Selection as a static method.

[Reinmar]

I'll reported a two followups:

• https://github.com/ckeditor/ckeditor5-engine/issues/652
• https://github.com/ckeditor/ckeditor5-list/issues/18