Convert Proposal directly to an Invoice

My jobs are small and usually 1 job = 1 invoice. I just send the invoice when I'm done working. I don't like setting up the invoices at the job's start, because the job is always changing. Instead, let me convert or copy a proposal to an invoice when I'm ready.

In my mind, the "Invoice" is just the next step in the proposal process. Estimate becomes a Proposal then becomes an Invoice.