Examples: GLFW+GL2: Moved glfw, gl2 files to root folder, split imgui_impl_glfw_gl2 into _opengl2 and reused imgui_impl_glfw

examples/imgui_impl_glfw.h

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+// ImGui GLFW binding with OpenGL3 + shaders
+// (GLFW is a cross-platform general purpose library for handling windows, inputs, OpenGL/Vulkan graphics context creation, etc.)
+// (GL3W is a helper library to access OpenGL functions since there is no standard header to access modern OpenGL functions easily. Alternatives are GLEW, Glad, etc.)
+
+// Implemented features:
+//  [X] User texture binding. Cast 'GLuint' OpenGL texture identifier as void*/ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
+//  [X] Gamepad navigation mapping. Enable with 'io.NavFlags |= ImGuiNavFlags_EnableGamepad'.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See main.cpp for an example of using this.
+// If you use this binding you'll need to call 4 functions: ImGui_ImplXXXX_Init(), ImGui_ImplXXXX_NewFrame(), ImGui::Render() and ImGui_ImplXXXX_Shutdown().
+// If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
+// https://github.com/ocornut/imgui
+
+struct GLFWwindow;
+
+IMGUI_API bool        ImGui_ImplGlfw_Init(GLFWwindow* window, bool install_callbacks);
+IMGUI_API void        ImGui_ImplGlfw_Shutdown();
+IMGUI_API void        ImGui_ImplGlfw_NewFrame();
+
+// GLFW callbacks (installed by default if you enable 'install_callbacks' during initialization)
+// Provided here if you want to chain callbacks.
+// You can also handle inputs yourself and use those as a reference.
+IMGUI_API void        ImGui_ImplGlfw_MouseButtonCallback(GLFWwindow* window, int button, int action, int mods);
+IMGUI_API void        ImGui_ImplGlfw_ScrollCallback(GLFWwindow* window, double xoffset, double yoffset);
+IMGUI_API void        ImGui_ImplGlfw_KeyCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
+IMGUI_API void        ImGui_ImplGlfw_CharCallback(GLFWwindow* window, unsigned int c);