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.

data/settings.gd

@@ -61,7 +61,6 @@ var api_password: String = ""
 ## the config file when it is not one of them.
 var servers: Array[String] = SERVERS.duplicate()
 var selected_server: int = 0
-var is_local: bool = false
 var is_debug: bool = false
 var stop_after_current: bool = true
 var auto_repeat_song: bool = true
@@ -70,7 +69,6 @@ var add_to_stats: bool = true
 var use_low_played_mode: bool = false
 var winning_score: int = 20
 var fullscreen: bool = false
-var play_local: bool = false
 var inspiration_list_speed: int = 1
 var hide_for_seconds: float = 5.0
 var hide_beginning: bool = false
@@ -87,6 +85,9 @@ var queue_ids: Dictionary[String, String] = {}
 ## The version the game last showed the About popup for. When it is not [member version],
 ## this version is new and the main window opens the About popup to show what is new.
 var last_seen_version: String = ""
+## The folder songs are played from instead of the server, picked in the settings, or null
+## to use the server. Kept for this session only.
+var music_folder: MusicFolder = null
 
 ## The game version, set in Project Settings > Application > Config > Version.
 ## Releases are made from it, see RELEASING.md.
@@ -220,6 +221,18 @@ func _saved_value(file: ConfigFile, section: String, key: String, default: Varia
 	return value
 
 
+## Whether songs come from a music folder instead of the server. Nothing but the
+## characters is asked from the server then.
+func is_offline() -> bool:
+	return music_folder != null
+
+
+## Whether a prepared quiz is played from a music folder. It has no winning score: the
+## game ends when every song has played.
+func is_prepared_quiz() -> bool:
+	return music_folder != null && music_folder.kind == MusicFolder.Kind.PREPARED_QUIZ
+
+
 func delete_children(node: Node) -> void:
 	for n: Node in node.get_children():
 		node.remove_child(n)