Implement optional music function arguments (issue 5023044)

[reinhold . kainhofer]

Regtest is missing (doesn't need to be a useful example, it just needs
to break if that functionality ever breaks!)

Also, does this work for cases like
  \relative c' c

Also, I suppose things like
  \myfunction [optional-pitch] pitch music
does not work due to the lookahead not looking too far, right?



http://codereview.appspot.com/5023044/diff/2001/lily/parser.yy
File lily/parser.yy (right):

http://codereview.appspot.com/5023044/diff/2001/lily/parser.yy#newcode1184
lily/parser.yy:1184: | EXPECT_MARKUP EXPECT_OPTIONAL function_arglist
function_markup_argument {
Can't we shorten this long list of alternatives somehow?

http://codereview.appspot.com/5023044/diff/2001/scm/music-functions.scm
File scm/music-functions.scm (right):

http://codereview.appspot.com/5023044/diff/2001/scm/music-functions.scm#newcode762
scm/music-functions.scm:762: "Helper macro for `ly:make-music-function'.
It's also a helper for ly:make-scheme-function...

http://codereview.appspot.com/5023044/diff/2001/scm/music-functions.scm#newcode792
scm/music-functions.scm:792: "
Here you should add a description how optional arguments are given! In
particular, the argX-type? is no longer valid in general.

http://codereview.appspot.com/5023044/diff/2001/scm/music-functions.scm#newcode801
scm/music-functions.scm:801: "
Same goes here.

http://codereview.appspot.com/5023044/