Better pure height approximations for stems.

Better pure height approximations for stems.

This makes several large changes to the Stem grobs' property list.

The following properties are deleted:

stem-end-position
stem-begin-position
length

Now, the only property that controls these three parameters is
Y-extent.

A new function, stem::length, has been added to allow for common
length overrides (i.e. cross-staff-stems) and is in the regtest
stem-length-override.ly.