appdata support

[Olivier Sessink]

Hi all,

I wrote a bluefish.appdata.xml file (see spec on
http://people.freedesktop.org/~hughsient/appdata/ ).

It's in subversion now, but not yet in the Makefile. On Linux it should
be installed in /usr/share/appdata/bluefish.appdata.xml, but on other
platforms it does not need to be installed. Can somebody help with the
Makefile?

Olivier