#25 use file associations on GNU/Linux

[Hans-Christoph Steiner]

On Windows and Mac OS X, Pd now opens files thru the
OS's file associations, on Windows using 'rundll32
dll.url.FileProtocolHandler' and on Mac OS X using the
'open' CLI program.

PDDP needs consistent file associations so that
[pddp_link] can be used to link to any file type within
a Pd patch.

A generic method might be to use firefox/mozilla as the
open method. GNOME and KDE have their own methods, and
these should be supported as well.