QTextDocument: improve import of DIV tags

<div>1<br/></div>2 was inserting two newlines between 1 and 2, while all
tested web browsers only insert one newline - as long as there is nothing
between the <br/> and the </div>.

This was the cause for extra newlines being inserted in KMail when
replying to HTML emails, such as those generated by gmail.

Change-Id: I5145d977701e68913264357bba22780e7cdd3f7d
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>