refactor/fix: calling Source.Selection.Select() was not working for FlatTree

[alexandrehtrb]

Calling Source.Selection.Select() was not working for FlatTree
Now it works, but only selecting one index at a time, I don't know how to select multiple using IndexPath...
Like, this works:

treeDataGrid.Source.Selection.Select(new IndexPath(1));
treeDataGrid.Source.Selection.Select(new IndexPath(2));

This doesn't work:

treeDataGrid.Source.Selection.Select(new IndexPath(1, 2));