Save fullscreen and read server, password and debug from config.ini

The settings the game saves itself, the quiz settings and now fullscreen, move
to settings.cfg (settings_debug.cfg in debug mode).

config.ini in the same folder is edited by hand: the server to start with, the
API password and debug mode. The game writes it with everything commented out
when it is missing and never changes it after that. IniFile reads it: plain
key = value lines, whole-line # or ; comments and optional quotes.

popups/settings_popup.gd

@@ -21,7 +21,7 @@ signal server_changed
 
 
 func _ready() -> void:
-	for server: String in Settings.SERVERS:
+	for server: String in Settings.servers:
 		select_server_button.add_item(server)
 	fullscreen_button.button_pressed = Settings.fullscreen
 	select_server_button.select(Settings.selected_server)
@@ -41,11 +41,7 @@ func _ready() -> void:
 
 
 func fullscreen() -> void:
-	Settings.fullscreen = !Settings.fullscreen
-	if Settings.fullscreen:
-		DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
-	if !Settings.fullscreen:
-		DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_WINDOWED)
+	Settings.set_fullscreen(!Settings.fullscreen)
 
 
 func local_play() -> void:
@@ -75,7 +71,7 @@ func set_selected_server(new_server: int) -> void:
 
 
 func _use_server(new_server: int) -> void:
-	Settings.default_path = Settings.SERVERS[new_server]
+	Settings.default_path = Settings.servers[new_server]
 	Settings.selected_server = new_server
 	GameLog.debug("Settings.default_path: " + Settings.default_path)
 	server_changed.emit()