Added Unicode window title support.

- Changed SFML API to receive an sf::String as the window title instead
  of a std::string (in Window::Window and Window::setTitle).
- Changed RenderWindow and WindowImpl APIs accordingly.
- Changed WindowImplWin32 to use a Unicode window title only if the
  target OS supports it.
- Changed WindowImplCocoa to always use Unicode window titles and added
  a utility function to Window/OSX/cpp_objc_conversion.mm.
- Changed WindowImplX11 to set the Unicode window title as part of the
  _NET_WM_NAME specification, which sadly is not part of the official X
  standard, but the closest anything can get. Still set regular ASCII
  title as fallback.