consider an `ExportWizard` state machine

[cfm]

After #1777, this is a stub ticket to observe that the ExportWizard is now a good candidate for trying out a QStateMachine. This could happen in either of two ways:

1. send-to-usb implements its own (non-Qt) state machine over its detection and export logic. The state it returns (see also: freedomofpress/securedrop-engineering#86) directly transitions securedrop-client's QStateMachine.

2. send-to-usb retains its single-shot linear logic. securedrop-client's QStateMachine transitions based on the status returned by send-to-usb.