Modify DataController#insertContent behaviour so inserting blocks into a list inserts new list items

[Reinmar]

TC:

1. Copy few paragraphs or other kind of blocks.
2. Place caret in a list item.
3. Paste the content.

Expected: The pasted paragraphs were converted to list items.

Actual: The default behaviour of the DataController#insertContent() algorithm is to insert new paragraphs, so the list will be split.

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[CKEditorBot]

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