Encourage A4 to page break before a tune.

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.

docsetup.A4.tex

@@ -38,5 +38,5 @@
 
 % Page appearance.
 \newcommand*{\pagestylemod}{}
-\newcommand*{\tunestart}{}
+\newcommand*{\tunestart}{\filbreak\vfill}
-\newcommand*{\tuneend}{\vfill}
+\newcommand*{\tuneend}{}