This enourages page breaks to happen before tune start, and not
between the tune image and any following wording.
Also move the vfill to the tune start. That way, if the break does not
happen, the vfill will, and the following tune will be spaced down
to the bottom of the page if it's the only other tune on the page.
Otherwise everything tends to clump at the top of the page.
A5 mostly did, but ran into trouble with longer comments on a tune.
This may give blank space after tunes where a long comment overflows
onto another page, but I think for now it's better to have each tune
at the top of the page.
Or rather, and this is the other change, in the middle of the page. Centre
the page content for A5 and Nook.
That way output where the colour isn't appropriate, such as Nook or the newly introduced
A4Lulu output (which is the A4 book but with white cover page and no end coloured page, and so
suitable for using as the content on Lulu, where the cover is separate) can omit the colour.
A lot of stuff was being duplicated between different page size. Consolidate where
possible, and in particular put as much into a single source file (dottes.tex) as
possible. Making a single Booke is now potentially quicker, as the tune pages
don't necessarily have to be regenerate every time.