LibDraw: Rename ImageLoader => ImageDecoder

ImageLoader was not the right name for this, as there is no loading
happening, only decoding. :^)

Libraries/LibDraw/ImageDecoder.cpp

@@ -0,0 +1,11 @@
+#include <LibDraw/ImageDecoder.h>
+#include <LibDraw/PNGLoader.h>
+
+ImageDecoder::ImageDecoder(const u8* data, size_t size)
+{
+    m_plugin = make<PNGImageDecoderPlugin>(data, size);
+}
+
+ImageDecoder::~ImageDecoder()
+{
+}