Add soft input support to Zebra/ZebraImpl

This includes making ZebraImpl observable, so that other objects can be
notified when the soft inputs change (via a SoftInputChangedEvent).
Merge branch '8.34'

Conflicts:
	uk.ac.gda.epics/src/gda/device/zebra/controller/Zebra.java
	uk.ac.gda.epics/src/gda/device/zebra/controller/impl/ZebraImpl.java