Convert vars into refs to avoid copy construction from const refs

src/gui/transferlistmodel.cpp

@@ -468,6 +468,6 @@ bool isDarkTheme()
 {
     const QPalette pal = QApplication::palette();
     // QPalette::Base is used for the background of the Treeview
-    const QColor color = pal.color(QPalette::Active, QPalette::Base);
+    const QColor &color = pal.color(QPalette::Active, QPalette::Base);
     return (color.lightness() < 127);
 }

src/webui/api/appcontroller.cpp

@@ -329,7 +329,7 @@ void AppController::setPreferencesAction()
 
         // Update deleted folders
         for (auto i = oldScanDirs.cbegin(); i != oldScanDirs.cend(); ++i) {
-            const QString folder = i.key();
+            const QString &folder = i.key();
             if (!scanDirs.contains(folder)) {
                 model->removePath(folder);
                 qDebug("Removed watched folder %s", qUtf8Printable(folder));

src/webui/api/synccontroller.cpp

@@ -434,9 +434,9 @@ void SyncController::maindataAction()
     data["torrents"] = torrents;
 
     QVariantHash categories;
-    const auto categoriesList = session->categories();
+    const auto &categoriesList = session->categories();
     for (auto it = categoriesList.cbegin(); it != categoriesList.cend(); ++it) {
-        const auto key = it.key();
+        const auto &key = it.key();
         categories[key] = QVariantMap {
             {"name", key},
             {"savePath", it.value()}