Revert "make state selection global"

This reverts commit 5714c9f6377d950e69cf91dbc211be7ba567892e.

STATE_SELECTION and TYPE_SELECTION were intentionally made class
attributes because they are later modified in other modules like
framework_agreement_requisition. That was a good idea, so I am reverting
that.

Specifically, this is blocking https://github.com/OCA/vertical-ngo/pull/10

Conflicts:
	purchase_rfq_bid_workflow/model/purchase_order.py