The current data/vagalume.desktop.in.in contains a deprecated category; while desktop-file-install and -validate still accept it as is, they do output warnings. Better to get it removed sooner rather than later.

The second commit updates the vagalume.desktop.in.in template to handle lastfm:// URLs (so doing xdg-open lastfm://globaltags/FOO would do the right thing; likewise, clicking on a lastfm:// link in the browser)

The Makefile target for creating desktop file is refactored to use inference rules, improving readability (and it would make it easier to support other desktop files in the future)