Handle <br> element as last child in <p>

[mfinger-irt]

It is possible to have a br element as last element inside a p. This leads to another line inside the imsc structure and therefore to a non-visible line inside the editor. If hints are activated the number of characters for this line is shown, but no line.

pWithBrEnd.ttml.txt

[andreastai]

This is indeed not intended. I add a bug label for it.