Added OS X impl of requestFocus and hasFocus

Signed-off-by: Foaly <foaly.f@web.de>
Signed-off-by: Jan Haller <bromeon@gmail.com>
Slightly adapted window focus documentation
Changed Window::requestFocus() return type from bool to void

Reasons:
* Consistent with other sf::Window methods
* User can test whether focus succeeded by subsequent hasFocus() call
* Implementation would have to call hasFocus() anyway on some systems

Also: minor code style change in Window::hasFocus()
Window focus: Added empty implementations for Android and iOS
Window focus on X11: check if window is viewable