SoundPlayer: Sync startup loop and show playlist settings in GUI

This fix syncs up the AudioPlayer's internal state for showing
playlist information with the AudioPlayer's GUI. Before, if the
AudioPlayer was opened with a playlist file (.m3u or .m3u8) it would
automatically show the playlist information in the GUI and set the
loop mode to playlist, but the menu options would be unchecked. In
order to hide the playlist information, the menu option would then
have to be toggled twice -- once on and again off.

Userland/Applications/SoundPlayer/Player.h

@@ -33,6 +33,7 @@ public:
     virtual ~Player() { }
 
     void play_file_path(String const& path);
+    bool is_playlist(String const& path);
 
     Playlist& playlist() { return m_playlist; }
     String const& loaded_filename() const { return m_loaded_filename; }