Revert "Issue 2990: \RemoveEmptyStaves in StaffGroup context crashes"

This reverts commit d40362adfc1c3b24f466b7570e58f92885751019.
Issue 3641: Keep only one Axis_group_engraver active

The internal context property axisEngraver is used for tracking the
currently active Axis_group_engraver.  This supercedes the solution
attempted in issue 2990.
Remove closed expressions before postevents and negative numbers
Eliminate closed argument lists before UNSIGNED and durations
Issue 3639/1: allow \repeat in optional arguments
Issue 3639/2: allow chords in optional arguments
Issue 3631: 2.17 does a worse job with vertical spacing and/or the page layout than 2.16

Looks for prebroken pieces of dead items in the pure relevant function.

Even if the item is dead, its prebroken pieces may not be.  We need to check
them and build them into the pure skylines of axis groups used by the
align interface.
Issue 2379: resetting autoBeaming after a cadenza

Letting measurePosition track the grace state of current_moment
unconditionally keeps it from moving into uncharted territory when
Timing.timing gets switched on or off during grace times.