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/AccelerometerGui.cpp

@@ -0,0 +1,51 @@
+/*----------------------------------------------------------------------------*/
+/* 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 "AccelerometerGui.h"
+
+#include <cstdio>
+
+#include <hal/Value.h>
+#include <imgui.h>
+#include <mockdata/AccelerometerData.h>
+
+#include "SimDeviceGui.h"
+
+using namespace halsimgui;
+
+static void DisplayAccelerometers() {
+  if (!HALSIM_GetAccelerometerActive(0)) return;
+  if (SimDeviceGui::StartDevice("BuiltInAccel")) {
+    HAL_Value value;
+
+    // Range
+    value = HAL_MakeEnum(HALSIM_GetAccelerometerRange(0));
+    static const char* rangeOptions[] = {"2G", "4G", "8G"};
+    SimDeviceGui::DisplayValue("Range", true, &value, rangeOptions, 3);
+
+    // X Accel
+    value = HAL_MakeDouble(HALSIM_GetAccelerometerX(0));
+    if (SimDeviceGui::DisplayValue("X Accel", false, &value))
+      HALSIM_SetAccelerometerX(0, value.data.v_double);
+
+    // Y Accel
+    value = HAL_MakeDouble(HALSIM_GetAccelerometerY(0));
+    if (SimDeviceGui::DisplayValue("Y Accel", false, &value))
+      HALSIM_SetAccelerometerY(0, value.data.v_double);
+
+    // Z Accel
+    value = HAL_MakeDouble(HALSIM_GetAccelerometerZ(0));
+    if (SimDeviceGui::DisplayValue("Z Accel", false, &value))
+      HALSIM_SetAccelerometerZ(0, value.data.v_double);
+
+    SimDeviceGui::FinishDevice();
+  }
+}
+
+void AccelerometerGui::Initialize() {
+  SimDeviceGui::Add(DisplayAccelerometers);
+}