Optimised OS X implementation regarding scaling factor

This also adds support for changing the screen profile or moving the window to another screen.
Improved OS X implementation

It makes sure the notifications sent to SFOpenGLView are only from its window.
Fixed OS X implementation of sf::Mouse::(get|set)Position

The code was not updated at all when support for retina display was introduced.
Merge branch 'bugfix/osx-implementation'