[BUG] Missing methods in 3.4 beta ( [er][et][sm][cd] [ENH] Made into change set; tested; recommend including )

[tomkoenig at mindspring.com]

This adds two simple methods to vertically or horizontally flip a
PolygonMorph.
There was no zipped change set; so I made one.
Manually tested this with variations on the following.
PolygonMorph arrowPrototype openInWorld; flipHAroundX: 90  
with Squeak 3.7a #5623
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