Fix for JENKINS-23152

Bulk of the fix is to move the event listener part out of the trigger and only reference the job by name and do a lookup of the job instance by name when needed.
This should let old instances of the jobs and triggers to be removed correctly and not have any duplicates cause trouble.
One known issue here is that it won't handle renames well.
A rename from the UI config page should be fine, but a reload from disk or item move could still cause old event listeners to be left in the list and potentially lose its corresponding Job that it should trigger.

So I'm thinking about making some kind of ItemReference class with an ItemListener somewhere that can update all the references somehow for the next step. But that could probably be in another change than this one.