QWizard: give all buttons an objectName

Only Commit, Finish and Cancel didn't have an object name, yet.
Also Extract Method on the switch statement, add a test, and
use QStringBuilder.

Task-number: QTBUG-29924
Reported-by: Leo Arias

Change-Id: I8c29606bc53e9d4caab631da2089e971a9da2d75
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
(cherry picked from qtbase/1ea191276ea49ce2334d21b1f4a2c66ee8889466)
QWizard: provoke 'enum value not handled in switch' warnings in object_name_for_button

This is strictly speaking a behavior change, since we're no longer returning
something for Stretch and NoButton, but - guessing here - that shouldn't matter.
Yet, better safe than sorry, thus submitted as a separate patch.

Change-Id: I20cb084147f8c9257ce37d1e87ea38febabec28d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
(cherry picked from qtbase/269590516d89a35ea3121366f7d4bf43b0d391ca)