Further work on signaling.
Merge remote-tracking branch 'github/master'

Conflicts:
	LC3Sim/src/lc3sim/core/ALU.java
	LC3Sim/src/lc3sim/core/BasicPropagator.java
	LC3Sim/src/lc3sim/core/BitWord.java
	LC3Sim/src/lc3sim/core/Listener.java
	LC3Sim/src/lc3sim/core/ListenerCallback.java
	LC3Sim/src/lc3sim/core/Register16.java