Handle including external PDF's

Use extension .ipdf to denote PDF's that should be saved to repository
as-is since they're taken from external resources and not generated from
LilyPond source. The Makefile copies these into the standard pdf/ output
folder and renames them to have .pdf extensions so they're properly
cleaned as needed.
Rename LilyPond files that are includes to .ily

This appears to be a LilyPond community convention to use for files that
are only intended to be used with \include and that usually provide
configuration or further command definitions.