[Review and testing] iOS examples (OpenGL, Window & Pong)

[Ceylo]

Hello!

We need your help both for code review and testing.
The PR: https://github.com/SFML/SFML/pull/1418

New stuff

The following GUI based examples are updated and now added to the generated Xcode project if you set SFML_BUILD_EXAMPLES to true:
- OpenGL
- Window
- Pong

These are the ones that don't use shaders.
The current "iOS" example is removed as it doesn't cover more usages than the above examples.

Review

The changes are mainly in CMake code, to fix linking and set a few required properties.
The examples C++ code also highlight a few behavior differences that we need to deal with: OpenGL vs OpenGL ES, different packaging of an iOS app, and mouse input vs touch input. If you see a more elegant way to deal with these, don't hesitate to tell!

Testing

Two aspects should be considered:
- you must be able to build & run the examples flawlessly from Xcode when targetting the simulator, same for a real device if you have a valid certificate from Apple. Note that there are still many issues with iOS implementation (aspect ratio when rotating, low-res display and graphical glitches), but these are out of scope in current PR.
- when performing an install of a desktop (not iOS!) build, the CMake files generated at PREFIX/lib/cmake/SFML must be identical with the ones generated by an install with current SFML master


Thanks for your time and feedbacks!