Planned font-related changes

[Jean Abou Samra]

As a follow-up to
[https://gitlab.com/lilypond/lilypond/-/merge_requests/1888#note_1341436372](https://gitlab.com/lilypond/lilypond/-/merge_requests/1888#note_1341436372)
I thought it might be helpful for reviewers to explain how
the somewhat numerous changes I am submitting and preparing
around fonts are organized and interrelate.

There are several independent fronts:

1. Improving the part of the code that takes an alist chain and
   chooses the basic selection method for the font: simplifying,
   decoupling font families and font sizes. Basically,
   [!1888](https://gitlab.com/lilypond/lilypond/-/merge_requests/1888).
   [!1917](https://gitlab.com/lilypond/lilypond/-/merge_requests/1917)
   has an important follow-up (renaming 'feta to 'music) that was
   left out of !1888 because it was already large/complex enough.

2. Make Pango font selection more flexible. The first step is
   [!1911](https://gitlab.com/lilypond/lilypond/-/merge_requests/1911).
   It depends on !1888, but only because there would otherwise
   be annoying merge conflicts; logically speaking, it is independent.
   There will be a second patch in this series to fix
   [#6539](https://gitlab.com/lilypond/lilypond/-/issues/6539).

3. Improve how LilyPond searches for music fonts in the file system.
   Goals are issues [#6485](https://gitlab.com/lilypond/lilypond/-/issues/6485)
   and [#6486](https://gitlab.com/lilypond/lilypond/-/issues/6486).
   A prerequisite is 
[!1903](https://gitlab.com/lilypond/lilypond/-/merge_requests/1903).
   There will be one or two more patches after that.

4. Get rid of the font-encoding property to simplify the internals. This is
   
[https://lists.gnu.org/archive/html/lilypond-devel/2023-03/msg00098.html](https://lists.gnu.org/archive/html/lilypond-devel/2023-03/msg00098.html).
   [!1914](https://gitlab.com/lilypond/lilypond/-/merge_requests/1914) is
   a prerequisite.

I have more ideas, mostly speculative; this is what is relatively settled at 
the moment.

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