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/RelayGui.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 "RelayGui.h"
+
+#include <cstdio>
+#include <cstring>
+
+#include <hal/Ports.h>
+#include <imgui.h>
+#include <mockdata/RelayData.h>
+
+#include "ExtraGuiWidgets.h"
+#include "HALSimGui.h"
+
+using namespace halsimgui;
+
+static void DisplayRelays() {
+  bool hasOutputs = false;
+  bool first = true;
+  static const int numRelay = HAL_GetNumRelayHeaders();
+  for (int i = 0; i < numRelay; ++i) {
+    bool forwardInit = HALSIM_GetRelayInitializedForward(i);
+    bool reverseInit = HALSIM_GetRelayInitializedReverse(i);
+
+    if (forwardInit || reverseInit) {
+      hasOutputs = true;
+
+      if (!first)
+        ImGui::Separator();
+      else
+        first = false;
+
+      bool forward = HALSIM_GetRelayForward(i);
+      bool reverse = HALSIM_GetRelayReverse(i);
+
+      ImGui::Text("Relay[%d]", i);
+      ImGui::SameLine();
+
+      // show forward and reverse as LED indicators
+      static const ImU32 colors[] = {IM_COL32(255, 255, 102, 255),
+                                     IM_COL32(255, 0, 0, 255),
+                                     IM_COL32(128, 128, 128, 255)};
+      int values[2] = {reverseInit ? (reverse ? 2 : -2) : -3,
+                       forwardInit ? (forward ? 1 : -1) : -3};
+      DrawLEDs(values, 2, 2, colors);
+    }
+  }
+  if (!hasOutputs) ImGui::Text("No relays");
+}
+
+void RelayGui::Initialize() {
+  HALSimGui::AddWindow("Relays", DisplayRelays,
+                       ImGuiWindowFlags_AlwaysAutoResize);
+  HALSimGui::SetDefaultWindowPos("Relays", 180, 20);
+}