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.

log.gd

@@ -1,3 +1,4 @@
+class_name LogPopup
 extends PopupWindow
 
 @onready