qprogressdialog has been the source of a number of problems in the past,
and the most recent issue is that it's showing the progress dialog
early, regardless of what the minimum duration is set to. since we're
already using our own logic for deciding when to show the dialog, it's
easier to move to a normal dialog box
also prevent timers from firing while a progress dialog is visible, or
if the refresh timer fires we end up with the same issue.
https://anki.tenderapp.com/discussions/beta-testing/949-anki-stops-when-field-is-added
