Examples: Add GLFW+Metal example

[bear24rw]

This example project aims to resolve the following issues with the existing apple example projects:

• example_apple_opengl2 depends on OpenGL which is now deprecated in macOS
• example_apple_metal uses an xcodeproj file to build instead of a Makefile like the rest of the examples
• example_apple_metal depends on imgui_impl_osx.mm which is not as feature complete as imgui_impl_glfw.cpp

[ocornut]

Merged, thank you very much Max!