Depending on the browser in different situations the root node itself is
selected and new text ends up in a text node on root level instead of a new
paragraph. This happens in:
* Firefox: after inserting a closed block like a horizontal rule
* Chromium: after inserting or selecting such a closed block
Now instead of inserting a paragraph directly after inserting an HR, the editor
simply checks for normal text input inside the root node and wraps the newly
written text with a paragraph (and moves the caret to the end of the paragraph
because chromium moves it to the beginning of the line)
