[KCompletion] Port away from deprecated methods in Qt 5.14
Summary:
the QComboBox::activated(QString) signal being deprecated
and optionally non-existent makes this interesting in subclasses trying
to emit it.
Test Plan:
-DQT_DISABLE_DEPRECATED_BEFORE=0x060000, make, ctest
and playing a bit with kcompletionuitest.
Reviewers: cfeck, dhaumann, aacid, vkrause
Reviewed By: vkrause
Subscribers: kde-frameworks-devel
Tags: Frameworks
Maniphest Tasks: T11553
Differential Revision: https://phabricator.kde.org/D23842