Add simulation GUI plugin

This uses Dear Imgui to provide a cross-platform integrated GUI for robot
simulation. The GUI provides fully integrated DS and joystick support so it's
not necessary to run the official DS.

simulation/halsim_gui/src/main/native/cpp/AnalogOutGui.cpp

@@ -0,0 +1,47 @@
+/*----------------------------------------------------------------------------*/
+/* 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 "AnalogOutGui.h"
+
+#include <cstdio>
+#include <cstring>
+#include <memory>
+
+#include <hal/Ports.h>
+#include <imgui.h>
+#include <mockdata/AnalogOutData.h>
+
+#include "SimDeviceGui.h"
+
+using namespace halsimgui;
+
+static void DisplayAnalogOutputs() {
+  static const int numAnalog = HAL_GetNumAnalogOutputs();
+  static auto init = std::make_unique<bool[]>(numAnalog);
+
+  int count = 0;
+  for (int i = 0; i < numAnalog; ++i) {
+    init[i] = HALSIM_GetAnalogOutInitialized(i);
+    if (init[i]) ++count;
+  }
+
+  if (count == 0) return;
+
+  if (SimDeviceGui::StartDevice("Analog Outputs")) {
+    for (int i = 0; i < numAnalog; ++i) {
+      if (!init[i]) continue;
+      char name[32];
+      std::snprintf(name, sizeof(name), "Out[%d]", i);
+      HAL_Value value = HAL_MakeDouble(HALSIM_GetAnalogOutVoltage(i));
+      SimDeviceGui::DisplayValue(name, true, &value);
+    }
+
+    SimDeviceGui::FinishDevice();
+  }
+}
+
+void AnalogOutGui::Initialize() { SimDeviceGui::Add(DisplayAnalogOutputs); }