Add callback handlers for LiveWindow (#2053)

Fixes #2223

wpilibc/src/main/native/cpp/livewindow/LiveWindow.cpp

@@ -104,10 +104,12 @@ void LiveWindow::SetEnabled(bool enabled) {
   // Force table generation now to make sure everything is defined
   UpdateValuesUnsafe();
   if (enabled) {
+    this->enabled();
   } else {
     m_impl->registry.ForeachLiveWindow(m_impl->dataHandle, [&](auto& cbdata) {
       cbdata.builder.StopLiveWindowMode();
     });
+    this->disabled();
   }
   m_impl->enabledEntry.SetBoolean(enabled);
 }

wpilibc/src/main/native/include/frc/livewindow/LiveWindow.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <functional>
 #include <memory>
 
 namespace frc {
@@ -22,6 +23,9 @@ class LiveWindow {
   LiveWindow(const LiveWindow&) = delete;
   LiveWindow& operator=(const LiveWindow&) = delete;
 
+  std::function<void()> enabled;
+  std::function<void()> disabled;
+
   /**
    * Get an instance of the LiveWindow main class.
    *