Replace Q_DECL_OVERRIDE with override

Summary: It's the modern way of doing things

Test Plan: builds

Reviewers: #calligra:_3.0, staniek

Reviewed By: #calligra:_3.0, staniek

Subscribers: staniek, Kexi-Devel-list

Tags: KProperty

Differential Revision: https://phabricator.kde.org/D25819