Adds parenthesized measure numbers at mid-bar breaks. (issue 5452057)

[reinhold . kainhofer]

http://codereview.appspot.com/5452057/diff/1/lily/bar-number-engraver.cc
File lily/bar-number-engraver.cc (right):

http://codereview.appspot.com/5452057/diff/1/lily/bar-number-engraver.cc#newcode107
lily/bar-number-engraver.cc:107: if (mp.main_part_ != Rational (0))
So, we now always print out the barnumber in parenthesis for mid-measure
breaks. The user has no chance to turn that off?

http://codereview.appspot.com/5452057/diff/1/lily/bar-number-engraver.cc#newcode110
lily/bar-number-engraver.cc:110: maybe_close_paren = ")";
Don't hardcode formatting settings, use a property instead.

http://codereview.appspot.com/5452057/