Added window methods to request and to check focus

Signed-off-by: Stefan Schindler <stefan@boxbox.org>
Signed-off-by: Jan Haller <bromeon@gmail.com>
Added OS X impl of requestFocus and hasFocus

Signed-off-by: Foaly <foaly.f@web.de>
Slightly adapted window focus documentation

Signed-off-by: Stefan Schindler <stefan@boxbox.org>