Avoid asynchronous task when calculating relative locations

The getRelativeLocation() asynchronously calculates the screen location
of the parent and child component. To calls should instead be done
synchronously, to avoid any race conditions caused by window changes
between those calls.

Amends bee6c0d

org.eclipse.wb.swing/src/org/eclipse/wb/internal/swing/utils/SwingUtils.java

@@ -314,8 +314,8 @@ public static Point getRelativeLocation(final Component parentComponent, final C
 			throws Exception {
 		try {
 			return runObjectLaterAndWait(() -> {
-				Point parentLocation = getScreenLocation(parentComponent);
-				Point childLocation = getScreenLocation(childComponent);
+				Point parentLocation = parentComponent.getLocationOnScreen();
+				Point childLocation = childComponent.getLocationOnScreen();
 				int relX = childLocation.x - parentLocation.x;
 				int relY = childLocation.y - parentLocation.y;
 				return new Point(relX, relY);