New line support in title, and hide title when empty

I added new line support in captions. When the title attribute of the link contains *real* new line chars, they are replaced with `<br>` in the HTML code of the caption `<span>`. It's quite a clean fix (works only in the remote case you place newlines in titles!) and does not affect anyway normal behavior.

Plus, I make title caption to disappear completely when final title string is empty (it does not make sense to display an empty black bar).