Add API for saving settings with default value

We should never actually write default values into the settings, because

- if the default value changes in a later Qt Creator version, the new
  default should automatically take effect if the user didn't change the
  value
- it senselessly grows the settings file

Add a QtcSettings class that extends QSettings by a
"setValueWithDefault" method, which does not write default values to the
settings, and actually removes the settingskey if the user switches back
to the default.

Use it at the places where we already do this manually.

Task-number: QTCREATORBUG-24762
Change-Id: Ia76414cb21e8521f3aeed1e37b43ae4fb3393ea3
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/qtcsettings.cpp

@@ -0,0 +1,54 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "qtcsettings.h"
+
+namespace Utils {
+
+/*!
+    \class Utils::QtcSettings
+    \inheaderfile utils/qtcsettings.h
+    \inmodule QtCreator
+
+    \brief The QtcSettings class is an extension of the QSettings class.
+
+    Use Utils::QtcSettings::setValueWithDefault() to write values with a
+    default.
+*/
+
+/*!
+    \fn template<typename T> void setValueWithDefault(const QString &key, const T &val, const T &defaultValue)
+
+    Sets the value of setting \a key to \a val. If \a val is the same as the \a
+    defaultValue, the settings key is removed instead. This makes sure that
+    settings are only written if actually necessary, namely when the user
+    changed them from the default. It also makes a new default value for a
+    setting in a new version of the application take effect, if the user did
+    not change the setting before.
+
+    \sa QSettings::setValue()
+*/
+
+} // namespace Utils