Allow multiple handlers per event hook

This allows comma-separated lists of event handlers, instead of a single one per event hook. Each listed handler is called in turn.  So, we can do several things at once if an event occurs, without having to write a single handler  to do it all or a wrapper script to act as an interface to several existing handlers.  The motivation for this was converting some existing NIWA suites, with existing custom event handlers, to use Rose (for which we also want to use  ```rose suite-hook```). 

@matthewrmshin et. al. - the functional change here - multiple event handlers per hook- is quite trivial and does not really add any complexity to cylc.  Most of this pull request is actually a significant simplification of the old event handler code.   If it turns out that you have a valid objection to having multiple handlers that I haven't thought of, I can easily back that out and keep the rest.