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/ImageDecoder.cpp

@@ -27,6 +27,8 @@
 #include <LibDraw/ImageDecoder.h>
 #include <LibDraw/PNGLoader.h>
 
+namespace Gfx {
+
 ImageDecoder::ImageDecoder(const u8* data, size_t size)
 {
     m_plugin = make<PNGImageDecoderPlugin>(data, size);
@@ -35,3 +37,4 @@ ImageDecoder::ImageDecoder(const u8* data, size_t size)
 ImageDecoder::~ImageDecoder()
 {
 }
+}