improving the transition between full-length and shortened stems. (issue4210051)

[lemniskata . bernoullego]

This patch handles transition between full length and shortened stems.
It follows Han-Wen suggestions and supports custom stem length and
custom staves.

proof-sheets:
Lily 2.13.47 output - http://www.sendspace.com/file/7fy9i4
this patch output - http://www.sendspace.com/file/tgmlfc
source code: http://www.sendspace.com/file/3pusct

http://codereview.appspot.com/4210051/