OSX build fails without include of QApplication

Summary:
OSX build fails with the following if QApplication is not included.

error: incomplete type 'QApplication' named in nested name
specifier
connect(QApplication::instance(), SIGNAL(applicationStateChanged(Qt::ApplicationState)), this,
^~~~~~~~~~~~~~
/usr/local/opt/qt/lib/QtGui.framework/Headers/qwindowdefs.h:81:7: note: forward declaration of 'QApplication'

Reviewers: lancaster, mutlaqja

Reviewed By: mutlaqja

Subscribers: KDE Edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D12357