Add callback handlers for LiveWindow (#2053)

Fixes #2223

wpilibj/src/main/java/edu/wpi/first/wpilibj/livewindow/LiveWindow.java

@@ -33,6 +33,9 @@ public class LiveWindow {
   private static boolean liveWindowEnabled;
   private static boolean telemetryEnabled = true;
 
+  private static Runnable enabledListener;
+  private static Runnable disabledListener;
+
   private static Component getOrAdd(Sendable sendable) {
     Component data = (Component) SendableRegistry.getData(sendable, dataHandle);
     if (data == null) {
@@ -46,6 +49,14 @@ public class LiveWindow {
     throw new UnsupportedOperationException("This is a utility class!");
   }
 
+  public static synchronized void setEnabledListener(Runnable runnable) {
+    enabledListener = runnable;
+  }
+
+  public static synchronized void setDisabledListener(Runnable runnable) {
+    disabledListener = runnable;
+  }
+
   public static synchronized boolean isEnabled() {
     return liveWindowEnabled;
   }
@@ -65,11 +76,17 @@ public class LiveWindow {
       updateValues(); // Force table generation now to make sure everything is defined
       if (enabled) {
         System.out.println("Starting live window mode.");
+        if (enabledListener != null) {
+          enabledListener.run();
+        }
       } else {
         System.out.println("stopping live window mode.");
         SendableRegistry.foreachLiveWindow(dataHandle, cbdata -> {
           cbdata.builder.stopLiveWindowMode();
         });
+        if (disabledListener != null) {
+          disabledListener.run();
+        }
       }
       enabledEntry.setBoolean(enabled);
     }