Makes all side-positioning based on skylines instead of boxes.

Makes all side-positioning based on skylines instead of boxes.

The major work is in side-position-interface.cc, with minor
modifications at several places in the code-base to adapt to this.

This allows for snugger positioning of horizontally-oriented
fingerings.

A side-effect of this patch is that side-positioning of all
cross-staff grobs delves into the element list of axis-groups
in order to better guess position, which results in less
collisions (for example, dynamics are less likely to collide
with cross-staff beams).