Fix KeyRelease event with CMD key pressed (close #381)

Solution based on pull request #401

Conflicts:
	src/sfml/window/OSX/SFApplication.m
	src/sfml/window/OSX/WindowImplCocoa.mm
Update logos for Cocoa example
Refactor SFOpenGLView and move code related to modifiers to a new file

Now SFKeyboardModifiersHelper.(h|mm) manage all the logic of modifier
flags and the state of the corresponding keys

Also, now modifiers key release events are correctly fired with
multiple SFML windows.

Conflicts:
	src/sfml/window/OSX/SFOpenGLView.h
	src/sfml/window/OSX/SFOpenGLView.mm