Examples: Downgraded Xcode project object version (50->48) to support Xcode 9.2 (last version supported on macOS 10.12) (#2133)

Examples: Using "dear imgui" terminology in all examples headers/comments + fix minor typo.

OSX: Added imgui_widgets.cpp to xcode project files.

Examples: Tweaked the main.cpp example structure for all examples. (There are a few hidden agendas here: 1) I would like to avoid encouraging people from using the implicit "Debug" window, and promote using Begin/End. In spite of my best attempt, there are a few feature of the upcoming docking system that cannot work 100% properly for the implicit Debug window, so future proof let's not put that feature in the spotlight too much for new users. 2) Moved dumb hardcoded positions into a single spot that can be replaced with a viewport relative position and not affect other demo windows. 3) Calling ShowDemoWindow before anything else, also for the benefit of a specific docking demo which will have an ordering constraint which is not really problematic in a real app but shouldn't be put forward in the demo.

Examples: Comments + shallow coding convention tweak to be consistent across examples and with imgui_impl_osx

Introduced a new Metal renderer implementation and a new example illustrating usage of Metal on macOS and iOS (partially addresses #1873)