Add a Rules Event for 'After a webform submission has been updated as a draft'

I'm working on a project that needs to use webform_rules to fire certain actions only on the final submission of a webform. The problem is that the 'insert as draft' event only works the first time the webform is submitted; the 'webform_rules_update' event currently ignores the is_draft value, which means that rules logic can't accommodate users making multiple 'Save as Draft' submissions.

The attached patch adds a 'webform_rules_update_as_draft' event to hook_rules_event_info().