LibGfx: Add support for "The Quite OK Image Format" (QOI)

The spec had its first stable release today, so I figured we should
support it as well!
As usual, by using the regular LibGfx image decoder plugin architecture,
we immediately get support for it everywhere: ImageViewer, FileManager
thumbnails, PixelPaint, and (with a small change in the subsequent
commit) even the Browser :^)

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -28,6 +28,7 @@ set(SOURCES
     PNGWriter.cpp
     PPMLoader.cpp
     Point.cpp
+    QOILoader.cpp
     Rect.cpp
     ShareableBitmap.cpp
     Size.cpp