Payment processor description field is blank for offline/backend event registrations

When creating a "live credit card" event registration through the admin interface, no description gets sent to the payment processor, even if the "event source" field is filled out.

I dug through CRM/Event/Form/Participant.php and it looks like this happens because _params['description'] is not populated until sometime after the payment is submitted to the processor.

Patch is attached to populate _params['description'] before submitting the payment, and also to provide a more meaningful default description if the admin doesn't fill out anything in the "event source" field on the form.