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/PWMGui.cpp

@@ -0,0 +1,66 @@
+/*----------------------------------------------------------------------------*/
+/* 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 "PWMGui.h"
+
+#include <cstdio>
+#include <cstring>
+
+#include <hal/Ports.h>
+#include <imgui.h>
+#include <mockdata/PWMData.h>
+
+#include "HALSimGui.h"
+
+using namespace halsimgui;
+
+static void DisplayPWMs() {
+  bool hasOutputs = false;
+  // struct History {
+  //  History() { std::memset(data, 0, 90 * sizeof(float)); }
+  //  History(const History&) = delete;
+  //  History& operator=(const History&) = delete;
+  //  float data[90];
+  //  int display_offset = 0;
+  //  int save_offset = 0;
+  //};
+  // static std::vector<std::unique_ptr<History>> history;
+  bool first = true;
+  static const int numPWM = HAL_GetNumPWMChannels();
+  for (int i = 0; i < numPWM; ++i) {
+    if (HALSIM_GetPWMInitialized(i)) {
+      hasOutputs = true;
+
+      if (!first)
+        ImGui::Separator();
+      else
+        first = false;
+
+      char name[32];
+      std::snprintf(name, sizeof(name), "PWM[%d]", i);
+      float val = HALSIM_GetPWMSpeed(i);
+      ImGui::Value(name, val, "%0.3f");
+
+      // lazily build history storage
+      // if (static_cast<unsigned int>(i) > history.size())
+      //  history.resize(i + 1);
+      // if (!history[i]) history[i] = std::make_unique<History>();
+
+      // save history
+
+      // ImGui::PlotLines(labels[i].c_str(), values.data(), values.size(),
+      // );
+    }
+  }
+  if (!hasOutputs) ImGui::Text("No PWM outputs");
+}
+
+void PWMGui::Initialize() {
+  HALSimGui::AddWindow("PWM Outputs", DisplayPWMs,
+                       ImGuiWindowFlags_AlwaysAutoResize);
+  HALSimGui::SetDefaultWindowPos("PWM Outputs", 910, 20);
+}