Added header sfml/widgets.hpp

"sfml/widgets.hpp" includes the GUI headers according the operating system.

Unix systems -> GTK -> gtk/gtk.h
Windows systems -> MFC -> windows.h & afxwin.h

See it like opengl.hpp which includes the right OpenGL version.
Implemented WidgetHandle

And renamed getWidgetHandler() to getWidgetHandle(), m_handler to m_handle.
Separated the interface from the implementation

It allows to add operating system specific back-end.