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.