Debian package Priority is now 'optional'

The previous 'extra' priority has been deprecated as of standards version
4.0.1; the expected priority for most packages is now 'optional'.

debian/control

@@ -1,9 +1,9 @@
 Source: darkradiant
 Section: editors
-Priority: extra
+Priority: optional
 Maintainer: Matthew Mott <orbweaver3d@gmail.com>
 Build-Depends: debhelper (>= 10), cmake (>= 3.12), pkg-config, libxml2-dev, libglew-dev, python-dev, libvorbis-dev, libopenal-dev, libalut-dev, libjpeg-dev, libftgl-dev, libwxbase3.0-dev, libwxgtk3.0-gtk3-dev, libsigc++-2.0-dev
-Standards-Version: 4.0.0
+Standards-Version: 4.0.1
 
 Package: darkradiant
 Architecture: any