runelite-api: add stopNow to cleanly shutdown the client

runelite-api/src/main/java/net/runelite/api/Client.java

@@ -140,6 +140,13 @@ public interface Client extends GameEngine
 	 */
 	void setGameState(GameState gameState);
 
+	/**
+	 * Causes the client to shutdown. It is faster than
+	 * {@link java.applet.Applet#stop()} because it doesn't wait for 4000ms.
+	 * This will call {@link System#exit} when it is done
+	 */
+	void stopNow();
+
 	/**
 	 * Gets the current logged in username.
 	 *