Fix: Link arrows go the wrong direction for multislots [6026]
Bug Number: 6026
Release notes (y/n): Yes
For Release Nums: 7.1.5 and 7.2

The problem was in the place in code where we compare the two slots at the 
ends of a Link to the slot sets (RootSelections) which define a Link Group.
If either of those two slots are SubSlots, we need to be looking at their 
parent MultiSlot when doing the RootSelection membership assessment.

See the change in method:
  void LinkGfxItem::setStyle (const LinkGroup*)

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