Departing from the original, rebranding as sfw::GUI

Changes:

• Renamed the namespace to sfw:: (from the old gui::), the root object is now sfw::GUI; renamed various other components.
• Reshuffled the source tree even more.
• Changed the API a bit (some cleanup, some improvements, perhaps some issues, too...).
• Decoupled the Widget API from the gfx. backend (SFML).

Auto-generated changelog: sfw-0.1.0...v0.2.0