Helper tools for more complex styling functions

[uliska]

The (define-styling-function) macro helps by figuring out the style-type, i.e. the way the modification has to be applied to the music. However, we need more support to help with creating more complex scenarios.

Apart from the application style-type we have other categories:

• grob target
  functions may have to apply different styles/overrides/elements to different grob types.
• custom properties
  functions may have to behave differently responding to additional properties. Typical example: c' \editorialMarkup emendation \with { type = addition } \( (span class is emendation, but there are different types of emendation which are visualized differently).

In fact this amounts to a three-dimensional matrix of combinations (tweak-slur-addition) that can't reasonably be realized within a complex conditional within a single music function. So we need to think about a way to put the different alternatives in lookup tables or the like.