allow editing of "event level" in participation records

Based on this forum thread: http://forum.civicrm.org/index.php/topic,14541.0.html

Currently, once a participation record has been created, there is no way to edit the Event Level / Event Fees for that record. In some cases, it's desirable to be able to edit this setting. For example, if the user wants to create "pending" registrations such as for invitees who have not yet registered; at the time of invitation, the user can only guess at the invitee's desired participation level, so he will want to be able to change this data when the user finally registers.

The attached patch makes this possible, as long as no payment record has been created for the given participation. This is meant to work with fees, discounted fee schedules, price sets, and fee adjustments via hook_civicrm_buildAmount().