Use Signal.Listener instead of Slot for Java 8 interop.
Use Signal.Listener instead of Slot for Java 8 interop.

When GWT 2.8.0 ships, I can probably move PlayN to Java 8 entirely (and build
in necessary Retrolambda Maven magic into the Android build), at which point I
can "fix" react.Slot and make it SAM-compatible. But for now, let's
Signal.Listener which is what react.Slot implements and what is actually needed
by the signal.

tinkerstudent/playn,rachelharvey/playn,playn/playn,tinkerstudent/playn,playn/playn,playn/playn,rachelharvey/playn