Terrible output for voiced notes with Fingering or StringNumber

[Thomas Morley]

The following code prints terrible output, for both cases, slightly
better if not in an event-chord

\new Staff <<
  \new Voice { \voiceOne < b'' -1 \2 > b'' -1 \2 }
  \new Voice { \voiceTwo < g' -2 \3 > g' -2 \3 }
>>

issue 3017 may be involved
https://sourceforge.net/p/testlilyissues/issues/3017/

I tried adding StringNumber to `direction-polyphonic-grobs' from
music-functions.scm but to no avail.
Looks like StringNumber doesn't respond to overrides/tweaks of
'direction, see below:

{
  %% default
  g' \3 -1
  %% voiceTwo
  \voiceTwo g' \3 -1 \oneVoice
  %% override doesn't work
  \once \override StringNumber.direction = #DOWN
  \once \override Fingering.direction = #DOWN
  g' \3 -1
  %% tweak doesn't work
  g'-\tweak direction #DOWN -\3 -\tweak direction #DOWN -1
  %% direction-modifiers work
  g' _\3 _1
  %% the verbose way works as well
  g'
   -\withMusicProperty #'direction #DOWN -\3
   -\withMusicProperty #'direction #DOWN -1
}

Adding direction-modifiers is a most simple work-around, I think we
should do better with default-settings, though.

Cheers,
  Harm