Add event for Job Item state change

Problem/Motivation

When a TMGMT Job Item changes its state, only TMGMT and the allocated processor plugin are aware of the change. If there were a more general event fired when the state changes, this could be used for custom and other contrib modules that aren't directly related to TMGMT. For example, an email of a state update could be sent out or the content from an accepted translation could be used for another language (as in fr => fr_ca).

Proposed resolution

Add an event to TMGMT and dispatch it in the JobITem::setState() method.

Remaining tasks

1. Add a JobItem State Change Event
2. Dispatch it from JobITem::setState()

User interface changes

None

API changes

None

Data model changes

None