[patch] (preliminary) bugfix: time signatures

[Juergen Reuter]

Hi!

Attached is a patch that should fix some problems of the time signature
engraver.  Some problems still survive.  I also renamed
style "old"->"neo_mensural".  The former "old6/8alt" font character is
currently not accessible since I had to remove the direct font access.
The former "C"-style is now the default behaviour.  It can be turned off
by setting style to "numbered", in which case only numbers are printed.

I think these changes are necessary, because the implementation so far was
a rather arbitrary, incomplete collection of time signature symbols.  I am
trying to orthogonalize all the different styles and to make combinations
of them possible.  This will also require the introduction of new
properties that qualify the style in more detail.  Note that I also
started to move program logic to scheme to simplify manipulation of the
logic.

See input/test/time.ly for comprehensive usage examples.

Greetings,
Juergen

patch_1.5.71.jr1.diff
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