format-metronome-mark and metronome-markup don't support styles other than default (issue 14807043)

[thomasmorley65]

Reviewers: ,

Message:
Please review.

Description:
format-metronome-mark and metronome-markup don't support styles other
than default

issue 3096

- introducing 'styled-metronome-markup' with the possibility to set
different styles for note-head and flag. Also, and currently quite
theoretical, it is now possible to use another font here. For now
the font-argument is used only to make
  \override MetronomeMark #'font-name = ...
possible.

- predefining several formatter and providing the possibility to let
them
customize by the user.

- adding a regtest for different 'metronomeMarkFormatter'.

- Limitation: There is still need to specify, what note-head/flag-style
should be taken.

Please review this at https://codereview.appspot.com/14807043/

Affected files (+162, -39 lines):
  A input/regression/metronome-mark-formatter.ly
  M scm/translation-functions.scm