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.

Userland/aplay.cpp

@@ -16,15 +16,18 @@ int main(int argc, char **argv)
     AClientConnection a_conn;
     a_conn.handshake();
     printf("Established connection\n");
-    AWavLoader loader;
-    const auto& buffer = loader.load_wav(argv[1]);
-    if (!buffer) {
-        dbgprintf("Can't parse WAV: %s\n", loader.error_string());
-        return 1;
+    AWavLoader loader(argv[1]);
+    printf("Loaded WAV\n");
+
+    for (;;) {
+        auto samples = loader.get_more_samples();
+        if (!samples) {
+            break;
+        }
+        printf("Playing %d sample(s)\n", samples->samples().size());
+        a_conn.play(*samples, true);
     }
 
-    printf("Playing WAV\n");
-    a_conn.play(*buffer);
     printf("Exiting! :)\n");
     return 0;
 }