Fixes: #1519

On this PR we are sending the backspace when the last character is being deleted and
when all the text is being deleted.

This improves our handling of list blocks when all content is deleted.

To test:

• Add a new list block
• Insert some lines
• Delete all content by selecting all the content
• Check that the placeholder content shows properly
• Write some characters
• Delete them one by one
• Check that when all are deleted the placeholder shows properly.
• Check the same behaviour on the other text blocks( paragraph, heading, blockquote)

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.