Issue 2199: align outside-staff grobs vertically in staff groups (issue 331650043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
Issue 2199: align outside-staff grobs vertically in staff groups

In any context in which a Vertical_align_engraver creates a
StaffGrouper, it also sees to it that all outside-staff grobs created
in the immediate context become y-axis children of the StaffGrouper.

Any such grobs which are later processed by the after-line-breaking
callback ly:side-position-interface::move_to_extremal_staff are moved
to the extremal staff of the group, not of the entire system.

Please review this at https://codereview.appspot.com/331650043/

Affected files (+206, -36 lines):
  A input/regression/bar-number-staff-group-context.ly
  A input/regression/metronome-marking-staff-group-context.ly
  M input/regression/rehearsal-mark-align-staff-context.ly
  A input/regression/rehearsal-mark-align-staff-group-context.ly
  M lily/include/staff-grouper-interface.hh
  M lily/include/system.hh
  M lily/side-position-interface.cc
  M lily/staff-grouper-interface.cc
  M lily/system.cc
  M lily/vertical-align-engraver.cc