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/ExtraGuiWidgets.cpp

@@ -0,0 +1,37 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017-2019 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 "ExtraGuiWidgets.h"
+
+namespace halsimgui {
+
+void DrawLEDs(int* values, int numValues, int cols, const ImU32* colors,
+              float size, float spacing) {
+  if (numValues == 0) return;
+
+  ImDrawList* drawList = ImGui::GetWindowDrawList();
+  const ImVec2 p = ImGui::GetCursorScreenPos();
+  float x = p.x + size / 2, y = p.y + size / 2;
+  int rows = 1;
+  for (int i = 0; i < numValues; ++i) {
+    if (i >= (rows * cols)) {
+      ++rows;
+      x = p.x + size / 2;
+      y += size + spacing;
+    }
+    if (values[i] > 0)
+      drawList->AddRectFilled(ImVec2(x, y), ImVec2(x + size, y + size),
+                              colors[values[i] - 1]);
+    else if (values[i] < 0)
+      drawList->AddRect(ImVec2(x, y), ImVec2(x + size, y + size),
+                        colors[-values[i] - 1], 0.0f, 0, 1.0);
+    x += size + spacing;
+  }
+  ImGui::Dummy(ImVec2((size + spacing) * cols, (size + spacing) * rows));
+}
+
+}  // namespace halsimgui