rework the new screen DB stuff:

- The tri-state --show actually did not make that much sense, which was particualrily obvious because of questions like
  "What should the default mode be?". We have -r/-e/-i for that. So replace --show by --silent, such that e.g.,
  `lilass -s -r mirror` will use the saved data and fall back to mirror mode, never showing a UI under normal circumstances.
  On the other hand, `lilass -s` *will* show a UI for unknown screens only.
- Rename last* -> previous*
- Sort resolutions by size only (not depending on preferred/previous resolution)
- Qt UI: pre-select previous/preferred resolution
- Remove some Debug output