enhancement request - new header field: movement

[address@hidden]

I've been wanting a "movement" field in LilyPond's headers.  I've managed to 
make something for myself that is working, more or less, but I'm sure it could 
be done much better by someone who actually knows what he's doing.  My idea is 
that it should be centered horizontally and be lower on the page than the other 
fields.  That is, the last header field before the printed music.  I think 
having the font the same as the title or maybe on size smaller would work well.

I think this would be of general utility for others as well.

-David