Improve OS X Implementation: performance and events handling

What this does:
* Optimised OS X implementation regarding scaling factor:
  - before: WindowImplCocoa's scaling function took ~20% of running time on a benchmark,
  - now: it takes << 1%.
* The scaling factor is adapted when the window is moved to another screen with another scaling factor.
  - Well, I hope so. I have no external monitor to test this...
* Fixed (un)focus events sent to all windows instead of only one.
* Fixed `sf::Mouse::(get|set)Position` functions.