[osis-core] line elements

[Chris Little]

Coming into the Dallas meeting we had <l> for linguistically meaningful 
(e.g. poetic) lines, and <milestone type="line"/> for orthographic line 
breaks.

Following Dallas, we had added <lb/>, also for orthographic line breaks.

I think this was intended just as a shorthand for <milestone 
type="line"/> since it would presumably be used frequently, if at all.

Did we deprecate <milestone type="line"/>, and if so, did we intend to 
remove it in this release?

--Chris