DataController#deleteContent should leave a paragraph if the entire content was selected

[pomek]

The whole ticket has been described here: https://github.com/ckeditor/ckeditor5-typing/issues/61

Agreed solution: https://github.com/ckeditor/ckeditor5-typing/issues/61#issuecomment-314699729

[pomek]

After a F2F talk with @Reinmar, we defined these TCs:

• <h1>[xx</h1><p>yy]</p> => <p>[]</p>
• <h1>[xx]</h1> => <h1></h1>
• <limit><h1>[xx</h1><p>yy]</p></limit> => <limit><p>[]</p></limit>
• no paragraph allowed in the limit/root => do not change anything (as it is now)
• all edge cases where selection isn’t really containing the whole content or when only a single element is selected:
  • e.g. <image></image><h1>[xx</h1><p>yy]</p> => <image></image><h1>[]</h1>
  • e.g. <h1>[<image></image>xx]</h1> => <h1>[]</h1>

Also, we agreed that $root could be added to Schema.limits to simplify limit checks.

[pomek]

=> means Backspace.