Add helper files for KAuth.

This creates a little executable, which can run with elevated privileges, and handles saving the files. This way the main application does not require root permissions.

src/helper/CMakeLists.txt

+
+add_executable(kcron_helper kcronhelper.cpp)
+target_link_libraries(kcron_helper PRIVATE KF5::AuthCore Qt5::Core KF5::Auth)
+
+ecm_qt_declare_logging_category(kcron_helper
+    HEADER kcm_cron_helper_debug.h
+    IDENTIFIER KCM_CRON_HELPER_LOG
+    CATEGORY_NAME org.kde.kcm.cron_helper
+    DESCRIPTION "kcm cron helper"
+    EXPORT KCRON_HELPER
+)
+
+# The following need to be disabled in debug builds, if lacking root permissions.
+# Build the project to generate the files, then install them manually:
+# /usr/share/dbus-1/system-services/ <- local.kcron.crontab.service
+# /usr/share/polkit-1/actions/ <- local.kcron.crontab.policy
+# /usr/share/dbus-1/system.d/ <- local.kcron.crontab.conf
+install(TARGETS kcron_helper DESTINATION ${KAUTH_HELPER_INSTALL_DIR})
+kauth_install_helper_files(kcron_helper local.kcron.crontab root)
+kauth_install_actions(local.kcron.crontab local.kcron.crontab.actions)

src/helper/README

+The contents of this folder, as well as a few tweaks
+regarding KAuth and polkit permissions were sourced 
+and modified from the GPL-licensed project Zeit by lolmu.
+
+The original project can be found here https://github.com/loimu/zeit.

src/helper/kcronhelper.cpp

+/* ========================================================================
+ *    Copyright (C) 2015-2021 Blaze <blaze@vivaldi.net>
+ *
+ *    This file is part of Zeit.
+ *
+ *    Zeit is free software: you can redistribute it and/or modify
+ *    it under the terms of the GNU General Public License as published by
+ *    the Free Software Foundation, either version 3 of the License, or
+ *    (at your option) any later version.
+ *
+ *    Zeit is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU General Public License for more details.
+ *
+ *    You should have received a copy of the GNU General Public License
+ *    along with Zeit.  If not, see <http://www.gnu.org/licenses/>.
+ * ======================================================================== *
+ *
+ *    This file was modified to fit into the project Kcron.
+ *    The same license terms apply.
+ *
+ * ======================================================================== */
+
+#include <QFile>
+
+#include "kcm_cron_helper_debug.h"
+
+#include "kcronhelper.h"
+
+ActionReply KcronHelper::save(const QVariantMap &args)
+{
+    qCDebug(KCM_CRON_HELPER_LOG) << "running actions";
+    QString source = args[QLatin1String("source")].toString();
+    QString destination = args[QLatin1String("target")].toString();
+    if (!QFile::remove(destination))
+        qCWarning(KCM_CRON_HELPER_LOG) << "can't remove file, it doesn't exist";
+    if (!QFile::copy(source, destination))
+        qCWarning(KCM_CRON_HELPER_LOG) << "can't write into the system file, something went wrong";
+    return ActionReply::SuccessReply();
+}
+
+KAUTH_HELPER_MAIN("local.kcron.crontab", KcronHelper)

src/helper/kcronhelper.h

+/* ========================================================================
+ *    Copyright (C) 2015-2021 Blaze <blaze@vivaldi.net>
+ *
+ *    This file is part of Zeit.
+ *
+ *    Zeit is free software: you can redistribute it and/or modify
+ *    it under the terms of the GNU General Public License as published by
+ *    the Free Software Foundation, either version 3 of the License, or
+ *    (at your option) any later version.
+ *
+ *    Zeit is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU General Public License for more details.
+ *
+ *    You should have received a copy of the GNU General Public License
+ *    along with Zeit.  If not, see <http://www.gnu.org/licenses/>.
+ * ======================================================================== *
+ *
+ *    This file was modified to fit into the project Kcron.
+ *    The same license terms apply.
+ *
+ * ======================================================================== */
+
+#pragma once
+
+#include <KAuth>
+
+using namespace KAuth;
+
+// usage of fully qualified type here renders the helper app fully ;) unusable
+// clazy:excludeall=fully-qualified-moc-types
+class KcronHelper : public QObject
+{
+    Q_OBJECT
+
+public Q_SLOTS:
+    ActionReply save(const QVariantMap &args);
+};

src/helper/local.kcron.crontab.actions

+[Domain]
+Name=Crontab Actions
+Policy=auth_admin
+Persistence=session
+
+[local.kcron.crontab.save]
+Name=Write Crontab
+Description=Write into the system crontab file
+Policy=auth_admin
+Persistence=session