pk: Make it possible to enable offline updates beyond the env var

Adds a setting that can be defined on kdeglobals such as:
```
[Software]
UseOfflineUpdates=true
```

BUG: 399369

libdiscover/backends/PackageKitBackend/PackageKitUpdater.cpp

@@ -28,6 +28,8 @@
 #include <QAction>
 #include <QSet>
 
+#include <KSharedConfig>
+#include <KConfigGroup>
 #include <KLocalizedString>
 
 int percentageWithStatus(PackageKit::Transaction::Status status, uint percentage)
@@ -151,13 +153,27 @@ void PackageKitUpdater::proceed()
     if (!m_proceedFunctions.isEmpty())
         processProceedFunction();
 #ifdef PKQT_1_0
-    else if (qEnvironmentVariableIsSet("PK_OFFLINE_UPDATE"))
+    else if (useOfflineUpdates())
         setupTransaction(PackageKit::Transaction::TransactionFlagOnlyTrusted | PackageKit::Transaction::TransactionFlagOnlyDownload);
 #endif
     else
         setupTransaction(PackageKit::Transaction::TransactionFlagOnlyTrusted);
 }
 
+bool PackageKitUpdater::useOfflineUpdates() const
+{
+    if (qEnvironmentVariableIsSet("PK_OFFLINE_UPDATE"))
+        return true;
+    KConfigGroup group(KSharedConfig::openConfig(), "Software");
+    return group.readEntry<bool>("UseOfflineUpdates", false);
+}
+
+void PackageKitUpdater::setUseOfflineUpdates(bool use)
+{
+    KConfigGroup group(KSharedConfig::openConfig(), "Software");
+    group.writeEntry<bool>("UseOfflineUpdates", use);
+}
+
 void PackageKitUpdater::start()
 {
     Q_ASSERT(!isProgressing());
@@ -195,7 +211,7 @@ void PackageKitUpdater::finished(PackageKit::Transaction::Exit exit, uint /*time
     m_backend->fetchUpdates();
     fetchLastUpdateTime();
 
-    if (qEnvironmentVariableIsSet("PK_OFFLINE_UPDATE")) {
+    if (useOfflineUpdates()) {
 #ifdef PKQT_1_0
         PackageKit::Daemon::global()->offline()->trigger(PackageKit::Offline::ActionReboot);
         Q_EMIT passiveMessage(i18n("Please restart the computer to finish the installation"));

libdiscover/backends/PackageKitBackend/PackageKitUpdater.h

@@ -83,6 +83,9 @@ class PackageKitUpdater : public AbstractBackendUpdater
         void fetchLastUpdateTime();
         void lastUpdateTimeReceived(QDBusPendingCallWatcher* w);
         void setupTransaction(PackageKit::Transaction::TransactionFlags flags);
+        bool useOfflineUpdates() const;
+        void setUseOfflineUpdates(bool use);
+
         QSet<QString> involvedPackages(const QSet<AbstractResource*>& packages) const;
         QSet<AbstractResource*> packagesForPackageId(const QSet<QString>& packages) const;