flip line-end-arrow-style on button icons (just to point upwards)
drawing direction of MetaContainment and Inheritance has been flipped

- drawing MetaContainment (when the user draws), it draws from the
metachild to the container
- drawing MetaInheritance (when the user draws), it draws from the child
to the parent
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the sheet still displays the connection the original way, i.e.
- MetaContainment 's connection {'src': container, 'dst': contained
metachild}
- MetaInheritance's connection: {'src': parent, 'dst': contained child}