Adjusts the arrow toggle buttons in the SimpleHierarchicalListMorph
(e.g. object explorer) so that they are properly centered, no matter
what the font size is.  Previously, the arrows would be in the
upper-left corner, which looked funny with larger font sizes (such as
New York 15 and larger).

This also makes it easier to swap in some other image/form for the
toggle button if desired.

For 2.9 alpha #3193.

- Doug Way
  dway at riskmetrics.com
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