Fix: (over-)long headlines break ugly and look as if they are two separate headlines

This patch reduces the `line-height` of all headlines to `1.25em` and adds the difference `0.5em` to their `margin-bottom`. I think this is needed to make headlines, which do not fit into the comment-column, look better. The former style looks as if there where two separate headlines following each other …

Hope you like it, :relieved: 
Stephan