SoundPlayer: Add start_new_file() to VisualizationWidget's API

This adds a new start_new_file() function to VisualizationWidget which
is called when the player starts a new file, passing the filename of the
file. This allows VisualizationWidget subclasses to do any setup needed
when a new file is started.

Userland/Applications/SoundPlayer/VisualizationWidget.h

@@ -15,6 +15,7 @@ class VisualizationWidget : public GUI::Frame {
 public:
     virtual void set_buffer(RefPtr<Audio::Buffer> buffer) = 0;
     virtual void set_samplerate(int) { }
+    virtual void start_new_file(StringView) { }
 
 protected:
     VisualizationWidget() = default;