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.cpp

@@ -0,0 +1,60 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "TimingGui.h"
+
+#include <cstdio>
+#include <cstring>
+#include <vector>
+
+#include <hal/HALBase.h>
+#include <imgui.h>
+#include <mockdata/MockHooks.h>
+#include <mockdata/NotifierData.h>
+
+#include "HALSimGui.h"
+
+using namespace halsimgui;
+
+static void DisplayTiming() {
+  int32_t status = 0;
+  uint64_t curTime = HAL_GetFPGATime(&status);
+
+  if (ImGui::Button("Run")) HALSIM_ResumeTiming();
+  ImGui::SameLine();
+  if (ImGui::Button("Pause")) HALSIM_PauseTiming();
+  ImGui::SameLine();
+  ImGui::PushButtonRepeat(true);
+  if (ImGui::Button("Step")) {
+    HALSIM_PauseTiming();
+    uint64_t nextTimeout = HALSIM_GetNextNotifierTimeout();
+    if (nextTimeout != UINT64_MAX) HALSIM_StepTiming(nextTimeout - curTime);
+  }
+  ImGui::PopButtonRepeat();
+  ImGui::PushItemWidth(ImGui::GetFontSize() * 4);
+  ImGui::LabelText("FPGA Time", "%.3f", curTime / 1000000.0);
+  ImGui::PopItemWidth();
+
+  static std::vector<HALSIM_NotifierInfo> notifiers;
+  int32_t num = HALSIM_GetNotifierInfo(notifiers.data(), notifiers.size());
+  if (static_cast<uint32_t>(num) > notifiers.size()) {
+    notifiers.resize(num);
+    HALSIM_GetNotifierInfo(notifiers.data(), notifiers.size());
+  }
+  if (num > 0) ImGui::Separator();
+  ImGui::PushItemWidth(ImGui::GetFontSize() * 4);
+  for (int32_t i = 0; i < num; ++i)
+    ImGui::LabelText(notifiers[i].name, "%.3f",
+                     notifiers[i].timeout / 1000000.0);
+  ImGui::PopItemWidth();
+}
+
+void TimingGui::Initialize() {
+  HALSimGui::AddWindow("Timing", DisplayTiming,
+                       ImGuiWindowFlags_AlwaysAutoResize);
+  HALSimGui::SetDefaultWindowPos("Timing", 5, 150);
+}