Fix Enter key not working for new Close button in Red Eye Reduce tool

Summary:
When df8cb1abf5ba was merged from stable to master, duplicating the
QDialogButtonBox in the Reduce Red Eye toolbar as done in
0c678376131d was not taken into account, so pressing Enter when
the Close button had focus for the initial helpDialogButtonBox
had no effect anymore.

This can be fixed by always operating on the buttons that are visible.

Test Plan:
Open the Reduce Red Eye tool and press Enter on the
Close, Reduce Red Eye and Cancel buttons as well as when
none of those buttons is focused.

Reviewers: muhlenpfordt

Reviewed By: muhlenpfordt

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D15014