Make it possible for PApplet subclasses to override exitActual.

core/src/processing/core/PApplet.java

@@ -4027,8 +4027,12 @@ public class PApplet extends Applet
     }
   }
 
-
-  void exitActual() {
+  /**
+   * Some subclasses (I'm looking at you, processing.py) might wish to do something
+   * other than actually terminate the JVM. This gives them a chance to do whatever
+   * they have in mind when cleaning up.
+   */
+  protected void exitActual() {
     try {
       System.exit(0);
     } catch (SecurityException e) {