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.

main/MainWindow.tscn

@@ -42,55 +42,55 @@ layout_mode = 1
 [node name="SearchPopup" parent="." unique_id=1661250375 instance=ExtResource("9_5ijvr")]
 visible = false
 layout_mode = 1
-offset_left = 56.0
-offset_top = 96.0
-offset_right = 56.0
-offset_bottom = 96.0
+offset_left = 384.0
+offset_top = 100.0
+offset_right = 384.0
+offset_bottom = 100.0
 
 [node name="SyncPopup" parent="." unique_id=182499699 instance=ExtResource("10_yxw0b")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
 offset_left = 210.0
-offset_top = 153.0
+offset_top = 220.0
 offset_right = 210.0
-offset_bottom = 153.0
+offset_bottom = 220.0
 
 [node name="QRPopup" parent="." unique_id=589733821 instance=ExtResource("15_qr_window")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
-offset_left = 720.0
-offset_top = 153.0
-offset_right = 720.0
-offset_bottom = 153.0
+offset_left = 735.0
+offset_top = 280.0
+offset_right = 735.0
+offset_bottom = 280.0
 
 [node name="ShortcutsPopup" parent="." unique_id=1513416242 instance=ExtResource("13_8eebo")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
-offset_left = 562.0
-offset_top = 160.0
-offset_right = 1262.0
-offset_bottom = 860.0
+offset_left = 610.0
+offset_top = 190.0
+offset_right = 1310.0
+offset_bottom = 890.0
 
 [node name="AboutPopup" parent="." unique_id=141261008 instance=ExtResource("16_about_window")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
 offset_left = 360.0
-offset_top = 153.0
+offset_top = 220.0
 offset_right = 360.0
-offset_bottom = 153.0
+offset_bottom = 220.0
 
 [node name="LogPopup" parent="." unique_id=1289881388 instance=ExtResource("14_26rwn")]
 unique_name_in_owner = true
 visible = false
 layout_mode = 1
-offset_left = 562.0
-offset_top = 132.0
-offset_right = 1262.0
-offset_bottom = 832.0
+offset_left = 610.0
+offset_top = 190.0
+offset_right = 1310.0
+offset_bottom = 890.0
 
 [node name="SoundtrackStatsPopup" parent="." unique_id=1213999001 instance=ExtResource("soundtrack_stats_popup_scene")]
 visible = false