Fix accessibility regression on the Dolphin Control button

9cd042a8 removed the text from the Control button without setting an
accessibleName property, which is a regression for screen-reader users.

This breaks the 19.12 string freeze, but since it's the same string we
were translating before 9cd042a8, hopefully it's not going to be too
much of a burden for translators.

CCBUG: 414271
CCMAIL: kde-i18n-doc@kde.org