Declare the ordering dependencies of listeners with before/after

Where a priority was picked to run before or after another listener and a
comment said so, the dependency is now declared next to the unchanged
priority. The compiled listener map is the same; a priority edit that
contradicts one of these dependencies now fails the build.

Packages that could still be installed with an older EventDispatcher get a
conflict, since those cannot read the named keys of getSubscribedEvents().