LibDraw: Put all classes in the Gfx namespace

I started adding things to a Draw namespace, but it somehow felt really
wrong seeing Draw::Rect and Draw::Bitmap, etc. So instead, let's rename
the library to LibGfx. :^)

Applications/DisplayProperties/DisplayProperties.h

@@ -71,6 +71,6 @@ private:
     Vector<String> m_wallpapers;
     RefPtr<GUI::Label> m_wallpaper_preview;
 
-    Size m_selected_resolution;
+    Gfx::Size m_selected_resolution;
     String m_selected_wallpaper;
 };