Various clean-ups in stems and beams. (issue 6584045)

[aleksandr . andreev]

http://codereview.appspot.com/6584045/diff/2001/ly/engraver-init.ly
File ly/engraver-init.ly (right):

http://codereview.appspot.com/6584045/diff/2001/ly/engraver-init.ly#newcode1150
ly/engraver-init.ly:1150: \override Beam #'positions =
#beam::get-kievan-positions
The only issue with putting this here is that if the user enters Kievan
notes without using the default KievanVoice context, the properties
don't get initialized. For example, this no longer works:

\relative c' {
   \override NoteHead #'style = #'kievan
   e8 [ g]
   a[ bes]
}

Either we have to initialize these properties when the style property of
the NoteHead is set to kievan or we have to change the documentation to
indicate that Kievan notation can only be used with the default
contexts. The latter option would require changing the snippets in the
documentation, among other things.

http://codereview.appspot.com/6584045/