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/SoundPlayerWidgetAdvancedView.cpp

@@ -206,6 +206,7 @@ void SoundPlayerWidgetAdvancedView::time_elapsed(int seconds)
 
 void SoundPlayerWidgetAdvancedView::file_name_changed(StringView name)
 {
+    m_visualization->start_new_file(name);
     m_window.set_title(String::formatted("{} - Sound Player", name));
 }