move export calls to their own QThread

[redshiftzero]

currently run_export_preflight_checks() and export_file_to_usb_drive() are running in the main thread. this thread is also running the main event loop for the GUI, and as such the UI won't be responsive until the calls return. we should move these two calls to their own QThread, and update the export dialog via a signal. once this is done, we can remove the manual QApplication.processEvents() call we added to ensure the dialog pops up before the first export call runs here.