add lux aeterna excerpt

This one-page excerpt was created from a very nice 1952
Breitkopf edition of Mozart's Requiem.
lux aeterna: change layout to match Breitkopf edition

Obviously they goal of this excerpt is to compare LilyPond's
engraving to the Breitkopf original, so here i set some general
settings to reflect that edition.
lux aeterna: beautify, level 2 - page layout