fix handling of new session

defer save state till after everything done in post_engine_init() is complete.
make quit work even when latency measurement is in progress
remove unused members and code from EngineControl dialog (related to different devices for input+output)
remove some configure time tests from libs/ardour regarding JACK which are no longer relevant
make use of measured latency values to set dialog controls, and use actual port latency to compute extra delay