Fix KDE Connect volume control
Summary: After this commit [1] in Qt, QAbstractEventDispatcher::instance()->metaObject()->className() returns "QXcbGlibEventDispatcher", so the glib event loop test fails
[1] http://code.qt.io/cgit/qt/qtbase.git/commit/?h=5.12&id=80b1cce0cfe5d8e527731b921fd5d0c1a007afd5
Test Plan: Play around with volume sliders in KDE Connect. Add/remove sinks
Reviewers: drosca
Reviewed By: drosca
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18276