Set Default Participant Role when loading back-office "Register Event Participant" form

Modify setDefaults() for CRM/Event/Form/Participant.php to set Participant Role to the value of civicrm_event.default_role_id if this is not NULL.

NOTES from Lobo on related issue (CRM-6111):
Setting role_id needs to trigger some javascript to pull in potentially custom fields set, and hence is a bit more complicated than just setting the default

Note that we are using eid=EVENT_ID_NUMBER for legacy reasons (and we are using eventId in an incredibly bad manner in an ajax call on that page for fees).

This whole form needs a cleanup and optimization.