Allow changing of notation fonts

Allow changing of notation fonts

This commit changes the behaviour of make-pango-font-tree in the
following ways:

- Make all arguments optional key/value pairs
  keeping the current default values (emmentaler/Century Schoolbook)
- Allow changing of music fonts too.

Currently alternative music fonts have to be present in the font
directory besides Emmentaler fonts, and they have to be manually installed.
But now there are a number of alternative and compatible fonts available.