Fix some compiler warnings

Summary:

• Fix order of member initialisation (compiling with -Wreorder)
• Use Q_UNUSED where needed
• QAtomicInteger::loadRelaxed() has been available since Qt 5.14

Test Plan: make && ctest

Reviewers: Frameworks, dfaure, apol

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27158