Fix enter behaviour in editables that cannot contain paragraphs

If the active editable does not allow paragraphs as its content (according
to http://www.w3.org/TR/2010/WD-html-markup-20101019/p.html, the element
must allow flow content), a linebreak is inserted instead of a paragraph
when pressing the Enter key, as if using shift-Enter.
Merge pull request #920 from maschonber/hotfix-linebreak-paragraph

Fix enter behaviour in editables that cannot contain paragraphs