issue 5393: Another fingering vs. accidental problem

issue 5393: Another fingering vs. accidental problem

	modified:   ../input/regression/finger-chords-accidental.ly
	modified:   ../input/regression/finger-chords-dot.ly
Existing regression tests supplemented by critical collision cases.

	modified:   ../lily/new-fingering-engraver.cc
- dots of all noteheads considered to avoid collisionts with fingerings
- Y-offset callback had been too late for correct X positioning:
  (A) Y-center stencil for FingeringColumn
  ---or---
  (B) Setting Y-offset (considering Y-offset set by user plus
                        self-alignment-Y and parent-alignment-Y)
      This is compatible to current behaviour (except for being in time)
- add_string function removed (obsolete, never used)

[Be-3, 2018-08-30 17:31:32]

Please review.
Thanks,
Torsten