Qt Designer: Support id-based translations

- Introduce a per-string id attribute and a global
  attribute indicating whether the form uses ids.
- Introduce a new "id" subproperty in the property editor
  which is used depending on the form setting.
- Add a settings to the form settings dialog.

[ChangeLog][Qt Designer] Qt Designer now supports id-based translations.

Task-number: QTBUG-34610
Change-Id: I9b0aa36fba0543960212a257e6aa2d9c23ab84c5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>