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.

quiz/quiz_settings_card.gd

@@ -4,7 +4,7 @@ extends BasePopup
 ## music player: how songs are played and hidden, the winning score and the inspiration
 ## list. It does not dim the quiz, so it can be changed during a song.
 ##
-## Every change is saved at once with [method Settings.save_quiz_settings].
+## Every change is saved at once with [method Settings.save_settings].
 ##
 ## The card's bottom edge is where the card is placed, just above the player bar, and
 ## it grows upward to fit its rows.
@@ -81,37 +81,37 @@ func increase_winning_score() -> void:
 func set_winning_score(new_score: int) -> void:
 	Settings.winning_score = maxi(new_score, 1)
 	score_label.text = str(Settings.winning_score)
-	Settings.save_quiz_settings()
+	Settings.save_settings()
 
 
 func lower_inspiration_speed() -> void:
 	Settings.inspiration_list_speed = maxi(Settings.inspiration_list_speed - 1, 1)
 	inspiration_speed_label.text = str(Settings.inspiration_list_speed)
-	Settings.save_quiz_settings()
+	Settings.save_settings()
 
 
 func increase_inspiration_speed() -> void:
 	Settings.inspiration_list_speed += 1
 	inspiration_speed_label.text = str(Settings.inspiration_list_speed)
-	Settings.save_quiz_settings()
+	Settings.save_settings()
 
 
 func lower_hide_for_seconds() -> void:
 	Settings.hide_for_seconds = maxf(Settings.hide_for_seconds - 1.0, 0.0)
 	hide_for_seconds_label.text = "%d" % Settings.hide_for_seconds
-	Settings.save_quiz_settings()
+	Settings.save_settings()
 
 
 func increase_hide_for_seconds() -> void:
 	Settings.hide_for_seconds = minf(Settings.hide_for_seconds + 1.0, MAX_HIDE_SECONDS)
 	hide_for_seconds_label.text = "%d" % Settings.hide_for_seconds
-	Settings.save_quiz_settings()
+	Settings.save_settings()
 
 
 # Sets the on/off quiz setting named [param setting] and saves the quiz settings
 func _save_toggle(on: bool, setting: StringName) -> void:
 	Settings.set(setting, on)
-	Settings.save_quiz_settings()
+	Settings.save_settings()
 
 
 func _on_hide_beginning_toggled(on: bool) -> void: