Add MAILBOX_CONTACT contact type

bramble-api/src/main/java/org/briarproject/bramble/api/contact/ContactType.java

@@ -28,7 +28,12 @@ public enum ContactType {
 	/**
 	 * A contact's mailbox
 	 */
-	CONTACT_MAILBOX(3);
+	CONTACT_MAILBOX(3),
+
+	/**
+	 * A contact of the mailbox owner connected to the mailbox
+	 */
+	MAILBOX_CONTACT(4);
 
 	private final int value;