Add default value option to event handler filterable values

src/main/java/org/nrg/xapi/model/event/EventClassInfo.java

...java/org/nrg/xapi/model/event/EventHandlerFilterInfo.java

src/main/java/org/nrg/xapi/rest/event/EventHandlerApi.java

...org/nrg/xnat/restlet/actions/AutomationBasedImporter.java

src/main/webapp/scripts/xnat/app/eventsManager.js

src/main/webapp/scripts/xnat/app/siteEventsManager.js