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

issue 3096

- introducing 'styled-metronome-markup' with the (currently quite
theoretical) possibility to use another font here. For now
the font-argument is used only to make
  \override MetronomeMark #'font-name = ...
possible.

- adding 'flag-style to define-grob-properties.scm, the
MetronomeMark-properties in define-grobs.scm and in text-interface.cc

- adding a regtest for different 'metronomeMarkFormatter'.

It's now possible to override MetronomeMark with 'style, 'flag-style and
'font-name to customize the appearance.
convert-ly: stricter pattern for one rule