Replace all occurrences of markup in notification.

fix_markup previously would only replace one occurrence of needle in the
notification string. So we'd end up with some markup's making it
through, if there was more than one of a particular kind.

Introduces new function string_replace_all and calls it from fix_markup.
updated changelog
Merge branch 'b_fix_markup-only_one_replace' of git://github.com/matcatc/dunst into fix_markup