Create GroupDissolvedEvent and react to it

Also react to incoming group invitations

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

@@ -0,0 +1,26 @@
+package org.briarproject.api.event;
+
+import org.briarproject.api.nullsafety.NotNullByDefault;
+import org.briarproject.api.sync.GroupId;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when a private group was dissolved
+ * by a remote creator.
+ */
+@Immutable
+@NotNullByDefault
+public class GroupDissolvedEvent extends Event {
+
+	private final GroupId groupId;
+
+	public GroupDissolvedEvent(GroupId groupId) {
+		this.groupId = groupId;
+	}
+
+	public GroupId getGroupId() {
+		return groupId;
+	}
+
+}