[CompositeStructureDiagram] Drag and drop of ports is fragile

The "drag and drop" of an existing port to the border of its owning class only works sometimes (if the mouse is exactly positioned on the border?). In most cases, DND does nothing with or without an error message. If the message is given, it indicates that ports can only be added to their owning classes or parts typed with these. Since the condition is met, the error message should indicate which owning class the tool has "seen", beside of fixing a minor typo ("can only be drag and drop")