Avoid calling overridable function from constructor

When a constructor calls an overridable function, it may allow an attacker to access the "this" reference prior to the object being fully initialized, which can in turn lead to a vulnerability.

Application -> getLocalPath
ProxyPanel -> get
AbortPanel -> get
RotatingBackgrounds -> makeEmpty

(Triggered by internal security audit and Fortify analysis.)

core/src/main/java/com/threerings/getdown/data/Application.java

@@ -36,7 +36,7 @@ import static java.nio.charset.StandardCharsets.UTF_8;
  * Parses and provide access to the information contained in the <code>getdown.txt</code>
  * configuration file.
  */
-public class Application
+public final class Application
 {
     /** The name of our configuration file. */
     public static final String CONFIG_FILE = "getdown.txt";