Make the text:line-break elements behave like <br>

Problem with the dipslay:block.
If we don't set a height for text:line-break, then there is no gaps
between sentences. As block element with no height doesn't take any space.
On the other hand if we do set a height for text:line-break, then all the
line breaks takes an empty line which is not exactly what we like. The
very first one should just end the line.
Merge pull request #138 from kevinlli/linebreakImprovement

Make the text:line-break elements behave like <br>