Give every script a class name and use it for typed references

The popups, the volume slider, the player name field and the main window get
a class_name, so the main window and the quiz hold them by their real type
instead of Control. The classes already use the names the files will get.

quiz.gd

@@ -33,10 +33,10 @@ var players_panel: PlayersPanel = $PlayersPanel
 var toolbar: QuizToolbar = $QuizToolbar
 
 @onready
-var settings_window: Control = $SettingsWindow
+var settings_window: SettingsPopup = $SettingsWindow
 
 @onready
-var winner_window: Control = %WinnerWindow
+var winner_window: WinnerPopup = %WinnerWindow
 
 @onready
 var song_controls: SongControls = $SongControls
@@ -48,7 +48,7 @@ var song_loader: SongLoader = $SongLoader
 var music_list_panel: MusicListPanel = $MusicListPanel
 
 @onready
-var new_character_select: Control = %NewCharacterSelect
+var new_character_select: CharacterSelectPopup = %NewCharacterSelect
 
 @onready
 var music_player_container: MusicPlayer = $MusicPlayer