Add NOT CHANGED operator to JQL

The CHANGED operator is JQL is very useful, but adding a NOT CHANGED operator would be very beneficial as well.

For example: 'status = "In Progress" AND status NOT CHANGED AFTER "-7d"' to find issues which are effectively stalled.

This is useful for teams implementing continous flow processes like KANBAN. Combined with the Greenhopper rapid board and quick filters a NOT CHANGED operator could help quickly hightlight things stalled on the board.

This behaviour can be simulated with current JQL by combining status WAS and status CHANGED, but this has to be specified for each status to observe, which is very hard to maintain when you have a more complex workflow.