[usability] Add currency_id in sale.order tree views and purchase.order tree view

I think that, when we display an amount, it doesn't make sense if we don't know the currency of that amount! This information is present in the tree views of account.invoice and hr.expense, but was lacking in the tree views of sale.order and purchase.order. This PR fixes this. I added the currency_id field before the amount field because it is in this order in the tree views of account.invoice and hr.expense.