Using .ily for included lilypond files

[Graham Percival]

In the large "DOC: Makefile" thread that nobody new is going to
read, there was a proposal to use .ily to indicate "setup"
lilypond files.

I quite like this suggestion, so I'm asking somebody to take the
next step: integrate this with lilypond.

- change (almost) everything in ly/ to .ily.
- update the internal "loading included files" stuff.
- update the docs (i.e. \include "gregorian.ly" should now
  just be "gregorian.ily")
- make a convert-ly rule.  Any of the built-in files (i.e.
  accordian.ly) should be changed automatically.  Since we
  still support including .ly, we don't need to print out
  a warning for other files.
- add a NEWS item.

Cheers,
- Graham