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/include/SimDeviceGui.h

@@ -0,0 +1,91 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <stdint.h>
+
+#include <hal/Types.h>
+#include <hal/Value.h>
+
+#ifdef __cplusplus
+#include <functional>
+
+#include <imgui.h>
+#endif
+
+extern "C" {
+
+void HALSIMGUI_DeviceTreeHide(const char* name);
+void HALSIMGUI_DeviceTreeAdd(void* param, void (*execute)(void*));
+HAL_Bool HALSIMGUI_DeviceTreeDisplayValue(const char* name, HAL_Bool readonly,
+                                          struct HAL_Value* value,
+                                          const char** options,
+                                          int32_t numOptions);
+HAL_Bool HALSIMGUI_DeviceTreeStartDevice(const char* label, int32_t flags);
+void HALSIMGUI_DeviceTreeFinishDevice(void);
+
+}  // extern "C"
+
+#ifdef __cplusplus
+
+namespace halsimgui {
+
+class SimDeviceGui {
+ public:
+  static void Initialize();
+
+  /**
+   * Hides device on tree.
+   *
+   * @param name device name
+   */
+  static void Hide(const char* name);
+
+  /**
+   * Adds device to tree.  The execute function is called from within the
+   * device tree window context on every frame, so it should implement an
+   * TreeNodeEx() block for each device to display.
+   *
+   * @param execute execute function
+   */
+  static void Add(std::function<void()> execute);
+
+  /**
+   * Displays device value formatted the same way as SimDevice device values.
+   *
+   * @param name value name
+   * @param readonly prevent value from being modified by the user
+   * @param value value contents (modified in place)
+   * @param options options array for enum values
+   * @param numOptions size of options array for enum values
+   * @return True if value was modified by the user
+   */
+  static bool DisplayValue(const char* name, bool readonly, HAL_Value* value,
+                           const char** options = nullptr,
+                           int32_t numOptions = 0);
+
+  /**
+   * Wraps ImGui::CollapsingHeader() to provide consistency and open
+   * persistence.  As with the ImGui function, returns true if the tree node
+   * is expanded.  If returns true, call StopDevice() to finish the block.
+   *
+   * @param label label
+   * @param flags ImGuiTreeNodeFlags flags
+   * @return True if expanded
+   */
+  static bool StartDevice(const char* label, ImGuiTreeNodeFlags flags = 0);
+
+  /**
+   * Finish a device block started with StartDevice().
+   */
+  static void FinishDevice();
+};
+
+}  // namespace halsimgui
+
+#endif  // __cplusplus