Implement SWITCH_MULTILEVEL_STOP_LEVEL_CHANGE handling for multilevel switches

Attempt to add full support for rollershutter operations. According to another user (maihacke) UP/DOWN are working, and with this change STOP also works, but after a STOP message the device hangs for a few seconds before becoming responsive again.

I am unsure if my implementation of the SWITCH_MULTILEVEL_STOP_LEVEL_CHANGE message is being done correctly, or if the UP/DOWN should actually be done using SWITCH_MULTILEVEL_START_LEVEL_CHANGE (which is currently not implemented).

Unfortunately I don't have any rollershutter devices so am unable to test, hence I am handing this over to someone who does.

Maihacke has created an issue (https://github.com/openhab/openhab/issues/693) which contains more details.