Add offline mode: play from a music folder instead of the server

Settings has a Music folder row. A folder with subfolders is played as
soundtracks: Next picks a random subfolder and a random song in it, the
subfolder is the answer's soundtrack and the file name without its type the
song. The inspiration list and Search show the subfolders.

A folder with songs and no subfolders is a prepared quiz, played once in order
(natural sort) or at random, showing only the song title. It has no winning
score: after the last song Next becomes Show winner, and the players with the
most points win, ties included.

While a folder is used nothing but the characters is asked from the server:
Sync, QR, Statistics, the update check and play counting are off. Characters
fall back to the cached pictures when the server can't be reached. mp3, ogg and
wav play; other files and broken ones are skipped.

The old local playback code, the hidden Open button and the hidden Local toggle
are removed. Next now waits for the song to start before preparing the next one.

player/music_player.gd

@@ -8,7 +8,6 @@ signal play_next_song
 
 var play_icon: Texture = preload("res://icons/play_icon_light.svg")
 var pause_icon: Texture = preload("res://icons/pause_icon_light.svg")
-var songs: Array = []
 var is_changing: bool = false
 var playback_position: float
 var stream: AudioStream
@@ -199,31 +198,19 @@ func play_song_object(song_object_no: int) -> void:
 	update_song_list.emit()
 
 
+## Plays a song to test the sound: one from the server, or a song that comes with the
+## game when playing from a music folder.
 func get_sound_test_song() -> void:
+	if Settings.is_offline():
+		sound_test_local()
+		return
 	var test_sound: Callable = func test_sound(body: PackedByteArray) -> void:
 		var sound: AudioStream = AudioStreamMP3.new()
 		sound.data = body
 		play_song(sound)
 	MusicServerClient.fetch_sound_test_song(test_sound)
 
-#####    LOCAL
-var local_path: String = '/Users/sebastian/ResilioSync/Sorterat_test/Metal Gear Solid 4 - Guns of the Patriots/2-16 Metal Gear Saga.mp3'
-
-
-func load_mp3(_path: String) -> AudioStream:
-	var file: FileAccess = FileAccess.open(_path, FileAccess.READ)
-	var sound: AudioStream = AudioStreamMP3.new()
-	sound.data = file.get_buffer(file.get_length())
-	return sound
-
-
-func play_local_song() -> void:
-	if songs:
-		local_path = songs[0]
-		GameLog.debug(local_path)
-		GameLog.debug(FileAccess.file_exists(local_path))
-		play_sound(load_mp3(local_path))
-
 
+# The sound test without a server: a song that comes with the game
 func sound_test_local() -> void:
-	play_sound(preload("res://sounds/01. Opening.mp3"))
+	play_song(preload("res://sounds/01. Opening.mp3"))