Ignore accidentals of the beam itself when guessing an initial configuration. (issue4450052)

[mtsolo]

http://codereview.appspot.com/4450052/diff/1/lily/beam.cc
File lily/beam.cc (right):

http://codereview.appspot.com/4450052/diff/1/lily/beam.cc#newcode1221
lily/beam.cc:1221:
This solution doesn't work for:


\new Staff {
 <<
   { \voiceOne s16 fis'' }
   \\
   { \voiceTwo e4 e }
   \\
   { \voiceFour
   f''8 e'' gis'' gis'' }

}

One interesting thing is that it is the stems, not the noteheads, that
are pushing this down.  If you remove stems from the
beam-collision-engraver, this problem does not arise.  Why would stems
do this but not note heads?

http://codereview.appspot.com/4450052/