Allow sf::Window to become a topmost window

This patch adds `sf::Window::setTopmost(bool)` to add the ability to make a window a topmost window, i.e. it won't be hidden under other non-topmost Windows.

This way SFML can be used to show small notification windows and similar things.

There's no downside to this Change; it's only missing a proper Cocoa implementation (Windows and X11 work fine for me).