* input/regression/alignment-vertical-spacing.ly: new file.

* input/regression/alignment-vertical-spacing.ly: new file.

* python/convertrules.py (conv): add rule.

* ly/music-functions-init.ly: rename outputProperty to overrideProperty

* lily/align-interface.cc (stretch_after_break): new
function. Read fixed-alignment-extra-space property.
(align_elements_to_extents): read alignment-extra-space property.

* scm/page-layout.scm (page-headfoot): annotate headsep and footsep.
(annotate-space-left): new function. Annotate space left on page.
(annotate-y-interval): new function.
(paper-system-annotate-last): new function. Annotate bottom-space.