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.

MainWindow.gd

@@ -1,3 +1,4 @@
+class_name MainWindow
 extends Control
 ## The main window: the top bar, the popups opened from it and the quiz below it.
 ##
@@ -11,22 +12,22 @@ var top_bar: TopBar = $TopBar
 var quiz: Quiz = $Quiz
 
 @onready
-var search_view: Control = $Search
+var search_view: SearchPopup = $Search
 
 @onready
-var sync_window: Control = %SyncWindow
+var sync_window: SyncPopup = %SyncWindow
 
 @onready
-var shortcuts_window: Control = %ShortcutsWindow
+var shortcuts_window: ShortcutsPopup = %ShortcutsWindow
 
 @onready
-var qr_window: Control = %QRWindow
+var qr_window: QRPopup = %QRWindow
 
 @onready
-var about_window: Control = %AboutWindow
+var about_window: AboutPopup = %AboutWindow
 
 @onready
-var log_window: Control = %Log
+var log_window: LogPopup = %Log
 
 @onready
 var statistic_popup: PopupPanel = $StatisticsPopupPanel