Refactor IceTipTreeNode to support use of a FormSet

[Rinzwind]

This pull request refactors IceTipTreeNode to support use of a FormSet (introduced in Pharo pull request #14998) for the icon and applies that to #repositoryModelsByGroup on IceTipRepositoryModel.

[Rinzwind]

There are two additional commits that I did not include in this pull request: commits a9abd57 and 9fd5505. Commit a9abd57 would change the SpImageTableColumn’s answer for #readObject: from a Form to a FormSet. That would not be a problem for #visitImageColumn: on SpMorphicTableCellBuilder as it just sends the answer #asMorph. But other uses of the SpImageTableColumn could break. I’m guessing that’s at least the case for ‘Spec-Gtk’.