Fix base attachments for slurs at linebreaks

[hanwenn]

http://codereview.appspot.com/109071/diff/39/46
File lily/phrasing-slur-engraver.cc (right):

http://codereview.appspot.com/109071/diff/39/46#newcode147
Line 147: if (!g->internal_has_interface (ly_symbol2scm
("span-bar-interface")))
this will work in some cases, but is specific to the ordering of the
prefatory material and the existence of the barline.  It would better to
catch an object that has references to all of the breakable items.

http://codereview.appspot.com/109071/diff/39/48
File lily/slur-scoring.cc (right):

http://codereview.appspot.com/109071/diff/39/48#newcode411
Line 411: }
fold this code - left and right are symmetric.

http://codereview.appspot.com/109071/diff/39/48#newcode577
Line 577: x = (d == LEFT) ? (x + 0.25) : (x - 0.25);
dont use conditional

  x += -d * 0.25;

http://codereview.appspot.com/109071/diff/39/49
File lily/slur.cc (right):

http://codereview.appspot.com/109071/diff/39/49#newcode197
Line 197: - Is there a better way to accomplish this?
I think you should first make a test case that contains all combinations
of key, slur, clef, barline etc. at linebreaks, and go from there -
hardcoding bar lines here seems too limited

http://codereview.appspot.com/109071