SoundPlayer: Start working on a GUI sound player application

This can play anything that AWavLoader can load (so obviously only WAV
files at the moment.)

It works by having a timer that wakes up every 100ms and tries to send
a sample buffer to the AudioServer. If our server-side queue is full
then we wait until the next timer iteration and try again.

We display the most recently enqueued sample buffer in a nice little
widget that just plots the samples in green-on-black. :^)

Applications/SoundPlayer/SampleWidget.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <LibGUI/GFrame.h>
+
+class ABuffer;
+
+class SampleWidget final : public GFrame {
+    C_OBJECT(SampleWidget)
+public:
+    explicit SampleWidget(GWidget* parent);
+    virtual ~SampleWidget() override;
+
+    void set_buffer(ABuffer*);
+
+private:
+    virtual void paint_event(GPaintEvent&) override;
+
+    RefPtr<ABuffer> m_buffer;
+};