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.

tests/integration/test_popups.gd

@@ -130,3 +130,25 @@ func test_toggle_opens_and_closes() -> void:
 	shortcuts.toggle()
 	assert_bool(shortcuts.visible).is_false()
 
+
+func test_every_popup_is_centred_in_the_window() -> void:
+	var window_centre: Vector2 = Vector2(1920, 1080) / 2
+	var popups: Array[BasePopup] = [_window.get_node("SoundtrackStatsPopup")]
+	for node_name: String in MAIN_POPUPS + QUIZ_POPUPS:
+		popups.append(_popup(node_name))
+	for popup: BasePopup in popups:
+		match popup.name:
+			"WinnerPopup":
+				popup.show_winner(_add_player("Crono"))
+			"CharacterSelectPopup":
+				popup.show_grid("Crono")
+			"SoundtrackStatsPopup":
+				popup.show_soundtrack(SoundtrackWithSongs.new())
+			_:
+				popup.open()
+		await _runner.simulate_frames(1)
+
+		var centre: Vector2 = popup.panel.get_global_rect().get_center()
+		assert_float(centre.distance_to(window_centre)).override_failure_message(
+				"%s is centred at %s" % [popup.name, centre]).is_less_equal(1.0)
+		popup.close()