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.

MusicPlayer.gd

@@ -25,7 +25,7 @@ var sound_effect_player: AudioStreamPlayer = $AudioSoundEffectPlayer
 var progress_slider: HSlider = $PlayerBar/MusicPlayerContainer/MusicPlayerSlider
 
 @onready
-var volume_slider: HSlider = $PlayerBar/MusicPlayerContainer/VolumeSlider
+var volume_slider: VolumeSlider = $PlayerBar/MusicPlayerContainer/VolumeSlider
 
 @onready
 var music_time_label: Label = $PlayerBar/MusicPlayerContainer/MusicTimeLabel