New bar line interface: take whichBar into account (issue 6695046)

[janek . lilypond]

Was it not for Graham Percival, to whom i dedicate all my code reviews,
i would have not find enough willpower to write a review!
(seriously!)
:)

So, Marc, the problem is that convert-ly rule didn't convert commands
involving whicBar context property?  Like, \set Staff.whichBar = \"|:\"
should be converted to set Staff.whichBar = \".|:\"?

If so, please write it a bit more explicitely in the commit message, and
reference Bar Line Interface's issue number and committish.  Other than
this, LGTM.

cheers,
Janek

http://codereview.appspot.com/6695046/