Improve TextEntered for OS X (close #377)

Code from pull request #401 was not used because some methods used
don't exist on 10.5 which is still supported by SFML 2.1.
Fix KeyRelease event with CMD key pressed (close #381)

Solution based on pull request #401
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.
Add support for right control key on OS X

Related to #401
Fix mouse moved event on OS X when dragging the cursor (close #277)
Remove dead code
Improve reactivity of setMousePosition on OS X (fix #290)
Improved mouse events on OS X (close #213, related to #277)
Fix typos
Improved mouse events on OS X regarding fullscreen mode
Add keypad return, equal and period keys support for OS X

Related to #401