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.

PlayerNameField.gd

@@ -1,3 +1,4 @@
+class_name PlayerNameField
 extends TextEdit
 
 signal enter_key_pressed