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.

main/main_window.gd

@@ -38,7 +38,7 @@ func _ready() -> void:
 	sync_popup.sync_started.connect(_on_sync_started)
 	sync_popup.sync_finished.connect(_on_sync_finished)
 	quiz.server_changed.connect(search_popup.get_list_of_soundtracks)
-	quiz.server_changed.connect(top_bar.check_for_update)
+	quiz.server_changed.connect(_on_music_source_changed)
 	quiz.event_logged.connect(log_popup.add_log_row)
 	statistics_view.soundtrack_selected.connect(soundtrack_stats_popup.show_soundtrack)
 
@@ -46,6 +46,14 @@ func _ready() -> void:
 	show_about_if_new_version()
 
 
+# After another server or a music folder was picked. Statistics need the server.
+func _on_music_source_changed() -> void:
+	top_bar.update_offline_state()
+	top_bar.check_for_update()
+	if Settings.is_offline() && statistics_view.visible:
+		show_tab(&"quiz")
+
+
 ## Opens the About popup, with what is new, the first time this version starts.
 func show_about_if_new_version() -> void:
 	if Settings.last_seen_version == Settings.version: