Interrupt all messaging sessions when the app starts shutting down.

This makes it more likely that connections will be closed cleanly.
However, the interrupt() method is currently ineffective for incoming
sessions as it won't interrupt a blocking read, e.g. when the packet
reader is waiting for a packet.

briar-api/src/org/briarproject/api/event/ShutdownEvent.java

@@ -0,0 +1,6 @@
+package org.briarproject.api.event;
+
+/** An event that is broadcast when the app is shutting down. */
+public class ShutdownEvent extends Event {
+
+}