Implement \eyeglasses as markup command

Implement \eyeglasses as markup command

-) implement \eyeglasses as markup command, using the postscript stored
   in the eyeglassesps variable
-) Use this markup command in musicxml2ly, too
-) Add regtest for \eyeglasses (in a \markup and as a markup attached to
   a note)
-) Remove the eyeglasses example from the postscript command and
   replace it by two rings (the stereo symbol) written in postscript