Permit selecting a diagonal of squares at once in Group.

Permit selecting a diagonal of squares at once in Group.

When filling in a cyclic subgroup or one of its cosets, I've often
found I wanted to set an entire diagonal to the same thing at once
(usually SW-NE, but the other way round too in non-abelian groups),
and it's a pain having to do that to each square individually.
Restricting multiple selection to diagonals makes it easy to get the
selection I really wanted.