HeaderLabel: add optional body text

[danirabbit]

As I'm working with more settings panes in particular I'm finding that I want to add description text below certain headers, but because of the built in extra spacing in headers it makes the layout kind of awkard. I'd like to be able to have spacing and sizing kind of specific for this case so we always have nice typography in this kinds of headers:

BEFORE

AFTER

With a little CSS: https://github.com/elementary/stylesheet/pull/1228