core: remove QSettings in qPref* and use a shared variable

Add qPrefPrivate class which contains one QSettings variable,
delete QSettings from qPref* class definitions

this secures there are only instance of QSettings
(QSettings needs to be in a QObject class to work)

Signed-off-by: Jan Iversen <jani@apache.org>

core/settings/qPrefPrivate.cpp

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "qPrefPrivate.h"
+
+qPrefPrivate::qPrefPrivate(QObject *parent) : QObject(parent)
+{
+}
+qPrefPrivate *qPrefPrivate::instance()
+{
+	static qPrefPrivate *self = new qPrefPrivate;
+	return self;
+}