add new change state "deferred"

in the discussion over abandoning stale changes, the requirement came up to be able to discern "really dead" from "postponed" changes.

"deferred" would be equal to "abandoned" for all practical purposes except a) presentation and b) filtering.

state transitions between "review in progress", "abandoned" and "deferred" should be possible each way.
(btw, there is an "abandon" button on staged changes. this is unreasonable.)

the "all" and "my" menu bars should get "deferred changes" default filters for convenience and psychological effect.