Parser should unwrap content from root container elements in list items

Currently, if a user manages to get a root container element such as a `p` or `div` element inside of a list item either by a browser issue like in #418 or by manually inserting HTML, it can cause several problems while editing the list. The content of these elements should be unwrapped by the parser, and line breaks should be used to properly split up the text within the list item instead.

Although having such root container elements within a list item technically isn't invalid HTML, they can very easily cause a list to break in the editor when working with them, so using text nodes with line breaks instead is a much more stable alternative.