Re: Issue 3918: Add \alternatingTimeSignatures (issue 97110045)

[lilyliska]

It took a while because in the meantime Peter Bjuhr joined efforts, and
we implemented the option of printing hyphens between the time
signatures. While I don't like this and wouldn't use it personally, it's
explicitly suggested by Gould.

Additional changes:
- change to \fractionList that only returns the stencil
  for overriding TimeSignature.stencil.
  This makes the function more versatile
  (also works when user moved Timing away from the
  Score context).
- Issue a compiler warning when the argument doesn't
  consist of a list of two-element lists.

One missing thing:
I think the appearance of the hyphen should be configurable but I'm not
sure about a consistent way to do so.
What I would be able to implement is checking if the user has defined
certain variables, and if he did use their content to define the
appearance. But I don't think that's clean design and interface. I would
prefer the user doing some \override, but I don't know how/if that can
be achieved.
But I would not make this feature (i.e. its lack) block the patch.

https://codereview.appspot.com/97110045/