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/AWavLoader.h

@@ -3,6 +3,7 @@
 #include <AK/AKString.h>
 #include <AK/RefPtr.h>
 #include <AK/StringView.h>
+#include <LibCore/CFile.h>
 
 class ABuffer;
 
@@ -13,10 +14,18 @@ class ByteBuffer;
 // Parses a WAV file and produces an ABuffer instance from it
 class AWavLoader {
 public:
+    explicit AWavLoader(const StringView& path);
     RefPtr<ABuffer> load_wav(const StringView& path);
     const char* error_string() { return m_error_string.characters(); }
 
+    RefPtr<ABuffer> get_more_samples();
+
 private:
-    RefPtr<ABuffer> parse_wav(ByteBuffer&);
+    bool parse_header();
+    CFile m_file;
     String m_error_string;
+
+    u32 m_sample_rate { 0 };
+    u16 m_num_channels { 0 };
+    u16 m_bits_per_sample { 0 };
 };