refactor example + adding imgui and imtui submodules

examples/third-party/imgui/imgui-extra/imgui_impl.h

@@ -0,0 +1,31 @@
+/*! \file imgui_impl.h
+ *  \brief Enter description here.
+ */
+
+#pragma once
+
+#if (defined(__EMSCRIPTEN__))
+#define IMGUI_IMPL_OPENGL_LOADER_GLEW
+#endif
+
+#include "imgui/imgui.h"
+
+struct SDL_Window;
+typedef void * SDL_GLContext;
+typedef union SDL_Event SDL_Event;
+
+IMGUI_API ImGuiContext* ImGui_Init(SDL_Window* window, SDL_GLContext gl_context);
+
+void IMGUI_API ImGui_Shutdown();
+void IMGUI_API ImGui_NewFrame(SDL_Window* window);
+bool IMGUI_API ImGui_ProcessEvent(const SDL_Event* event);
+
+void IMGUI_API ImGui_RenderDrawData(ImDrawData* draw_data);
+
+bool IMGUI_API ImGui_CreateFontsTexture();
+void IMGUI_API ImGui_DestroyFontsTexture();
+bool IMGUI_API ImGui_CreateDeviceObjects();
+void IMGUI_API ImGui_DestroyDeviceObjects();
+
+void IMGUI_API ImGui_SaveState(int id);
+void IMGUI_API ImGui_LoadState(int id);