Simplify filename_to_display()

Use g_filename_to_utf8() to convert filenames from the GLib filename
encoding to UTF-8 for display to the user.
Use GIO to launch audio player process

Cut out platform-specific code to spawn a child process and instead use
g_app_info_launch().
Use GIO to launch open with process

Cut out platform-specific code to spawn a child process and instead use
g_app_info_launch().