alternating-time-sig: initial version

Print alternating time signatures and make the first given the current time.
Accepts a list with _two_ pairs.

Apart from possible improvements in coding this should accept _any_ number
of pairs to print any number of timesigs.
alternating-time-sig: let it use lists instead of pairs

I think this is less typing and it will be simpler to make the change to
the column lists later.
alternating-time-sig: start generating the column-list (WIP)

Try to generate the markup-list for columns through Scheme.
Seems to work (except effective time). Save snapshot
alternating-time-sig: Use a list of lists as argument

This makes the function generic to accept an arbitrary number of
time signature items.
alternating-time-signatures: Make example more complex

This is not necessary for the given task, but shows off LilyPond's
rhythmic-metrical power
alternating-time-signatures: use \center-column

... to align the numbers better.
Merge branch 'origin/alternating-time-sig'

Working version of \alternatingTimeSignatures.
I have not updated it to our snippets structure because I still
hope to include it directly into LilyPond.
alternating-time-signatures: make example clearer

now "reverts" to a \time that hasn't been in the alternating group