ScreenPool: Store primary connector separately

* Store the primary connector in a dedicated config key instead of
  making slot 0 do double-duty.  This prevents setPrimaryConnector()
  from renumbering the connector list and 'losing' existing
  panel/desktop settings.
* Add a helper function to generate connector names instead of
  using QScreen::name() everywhere.  The helper function defaults to a
  QScreen::manufacturer()/model()/serialNumber() tuple where available
  with a fall-back to the original name() logic.

BUG: 427278