Replace Xlib with XCB for linux sfml-window

This patch closes #200

I've ported the linux implementation of sfml-window to xcb as far as it's possible.
As already said in the dicussion of the issue, this is not possible for GLX.
Hence this implementation uses Xlib/Xcb to mix calls of both libraries.

I know this is a very huge commit, so take your time. ;)