Add an expert font tree interface (issue 108700043 by address@hidden)

[pkx166h]

Fails make check

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regression/font-expert-selection.ly

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Processing
`/tmp/build-lilypond-autobuild/out/lybook-testdb/73/lily-16d682d4.ly'
Parsing...
Renaming input to:
`/tmp/lilypond-autobuild/input/regression/font-expert-selection.ly'
/tmp/lilypond-autobuild/input/regression/font-expert-selection.ly:84:31:
error: syntax error, unexpected \lyricmode,
expecting \sequential or \simultaneous or << or '{'
    \new Lyrics \lyricsto "v"
                              \lyricmode { Foo -- bar baz xyz -- zy! }
/tmp/lilypond-autobuild/input/regression/font-expert-selection.ly:80:3:
error: errors found, ignoring music expressio
n

  <<
/tmp/lilypond-autobuild/input/regression/font-expert-selection.ly:99:1:
error: Unfinished main input

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https://codereview.appspot.com/108700043/