Missing shortcut for "next" and "back" buttons in wizards

1. Build trivialwizard example ( qtbase\examples\widgets\dialogs\trivialwizard\trivialwizard.pro ).
2. Start the example and click "Next".
3. Press Alt+Left, the keyboard shortcut for the "back" button.
   Nothing happens.
4. Press Alt+Right, the keyboard shortcut for the "next" button.
   Nothing happens.
5. Click the "Back" button in the upper left corner of the wizard.
   The wizard goes back to the first page.

The shortcut which works in native dialogs should work in Qt dialogs, too.

Found while updating Creator's Squish test tst_create_proj_wizard: https://codereview.qt-project.org/71942