Fix #943 (input/regression/slur-broken-trend.ly broken) (issue203054)

[pnorcks]

http://codereview.appspot.com/203054/diff/1/2
File lily/system.cc (right):

http://codereview.appspot.com/203054/diff/1/2#newcode310
lily/system.cc:310: {
Hi Neil,

I'm not really sure about the order of operations here, but I noticed
something else:

Removing the 'after-line-breaking callback from BarNumber also appears
to fix the problem.

Specifically, the slur regtest is fixed and
"page-spacing-rehearsal-mark.ly" is unaffected.

Is the 'after-line-breaking callback for BarNumber necessary?

http://codereview.appspot.com/203054/show