Add timing window to simulation GUI

This shows the FPGA time and notifier timing, and has buttons to
start/pause/step the simulation.

The GUI also pauses DS new data notifications when paused.  This could be
done globally instead by blocking NotifyNewData at the HAL level?

simulation/halsim_gui/src/main/native/cpp/TimingGui.h

@@ -0,0 +1,17 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+namespace halsimgui {
+
+class TimingGui {
+ public:
+  static void Initialize();
+};
+
+}  // namespace halsimgui