Enforce HTMLParagraph in paragraphs without any block-level styles.

[diegoreymendez]

Description:

Right now we're allowing editing of text outside of block-level elements. We should make sure that whenever a paragraph doesn't have any block-level (ie: paragraph-level) styles, we're at least enforcing the HTMLParagraph style.

Details:

Character lineFeed is used for representing paragraph delimiters (such as a block-level element's ending).
Character lineSeparator is used for representing <br> nodes.

The problem with allowing edition outside of block-level (ie: paragraph-level) styles is that we can no longer represent paragraph delimiters easily in HTML mode. The distinction becomes harder to enforce.