ImageDecoder: Handle decoding of images in an asynchronous manner

ImageDecoder now queues up image decoding requests and returns the
images back to the caller later. ImageDecoderClient has a new
promise-based API that allows callers to attach their own resolve/reject
handlers to the responses from ImageDecoder.

Userland/Services/ImageDecoder/ImageDecoderClient.ipc

@@ -1,6 +1,7 @@
-#include <LibCore/AnonymousBuffer.h>
 #include <LibGfx/ShareableBitmap.h>
 
 endpoint ImageDecoderClient
 {
+    did_decode_image(i64 image_id, bool is_animated, u32 loop_count, Vector<Gfx::ShareableBitmap> bitmaps, Vector<u32> durations, Gfx::FloatPoint scale) =|
+    did_fail_to_decode_image(i64 image_id, String error_message) =|
 }