Remove group as allowed child of tree

[mcking65]

Closes #2014

Removes group as an allowed accessibility child of tree to fix inconsistency with definition of treeitem. Direct children of tree can only be parent nodes or end nodes. Leaf nodes must be a child of a group that is a child of a parent node treitem. Even in a virtualized tree, if the elements that represent the ancestry of rendered nodes were not present in the DOM, the tree would be inoperable.

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