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/qPrefDisplay.cpp

@@ -1,11 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "qPref.h"
-#include "qPref_private.h"
+#include "qPrefPrivate.h"
 #include "core/subsurface-string.h"
 
 #include <QApplication>
 #include <QFont>
 
+static const QString group = QStringLiteral("Display");
+
 qPrefDisplay::qPrefDisplay(QObject *parent) : QObject(parent)
 {
 }