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. :^)

Libraries/LibDraw/DisjointRectSet.cpp

@@ -26,6 +26,8 @@
 
 #include <LibDraw/DisjointRectSet.h>
 
+namespace Gfx {
+
 void DisjointRectSet::add(const Rect& new_rect)
 {
     for (auto& rect : m_rects) {
@@ -69,3 +71,5 @@ void DisjointRectSet::shatter()
         swap(output, m_rects);
     } while (pass_had_intersections);
 }
+
+}