Fixed problems with the .desktop file

This change fixes the following error produced by desktop-file-install:
/usr/share/applications/milkytracker.desktop: error: value "AudioVideo;Audio;AudioVideoEditing;;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"

It also adds "%f" to Exec to allow passing a filename as an argument, and adds Sequencer to the list of categories.