Centre every popup in the window

Most popups sat where they had been placed by hand, up to 328 px off.
They are now all centred, and test_popups.gd checks it for every popup.

quiz/Quiz.tscn

@@ -93,28 +93,28 @@ offset_bottom = 975.0
 [node name="SettingsPopup" parent="." unique_id=1323246978 instance=ExtResource("11_k62u5")]
 visible = false
 layout_mode = 1
-offset_left = 720.0
-offset_top = 153.0
-offset_right = 720.0
-offset_bottom = 153.0
+offset_left = 710.0
+offset_top = 220.0
+offset_right = 710.0
+offset_bottom = 220.0
 
 [node name="CharacterSelectPopup" parent="." unique_id=1679856322 instance=ExtResource("6_dhrvg")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
-offset_left = 405.0
-offset_top = 165.0
-offset_right = 1557.0
-offset_bottom = 805.0
+offset_left = 315.0
+offset_top = 208.0
+offset_right = 1467.0
+offset_bottom = 848.0
 
 [node name="WinnerPopup" parent="." unique_id=999999999 instance=ExtResource("17_winner_window")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
 offset_left = 160.0
-offset_top = 100.0
+offset_top = 80.0
 offset_right = 160.0
-offset_bottom = 100.0
+offset_bottom = 80.0
 
 [node name="DebugLabel" type="Label" parent="." unique_id=1959221257]
 visible = false