REF: Move _shallow_copy from SelectionMixin to Window

[jbrockmendel]

SelectionMixin._shallow_copy is only used by Window/Rolling/... subclasses. Since SelectionMixin is mixed into NDFrame, it would be nice to narrow down the namespace. Thoughts @mroeschke ?

[mroeschke]

Make sense to me if it's only used by Window/Rolling. Happy to move it there.