Prevents cross-staff Stems from colliding with articulations.

Prevents cross-staff Stems from colliding with articulations.

Does so by creating a StemStub grob for each Stem whose only effect on
spacing comes from its extra-spacing-height, which blocks articulations
from bumping into the Stems.  Beamed stems cannot be considered in
extra-spacing-height because they trigger a circular dependency in the
calculation of direction.