Audio: Make basic streaming WAV playback work.

I had to solve a bunch of things simultaneously to make this work.
Refactor AWavLoader to be a streaming loader rather than a one-shot one.
The constructor parses the header, and if everything looks good, you can
repeatedly ask the AWavLoader for sample buffers until it runs out.

Also send a message from AudioServer when a buffer has finished playing.
That allows us to implement a blocking variant of play().

Use all of this in aplay to play WAV files chunk-at-a-time.
This is definitely not perfect and it's a little glitchy and skippy,
but I think it's a step in the right direction.

Libraries/LibAudio/ASAPI.h

@@ -5,6 +5,7 @@ struct ASAPI_ServerMessage {
         Invalid,
         Greeting,
         PlayingBuffer,
+        FinishedPlayingBuffer,
     };
 
     Type type { Type::Invalid };