kcm/kwinrules: Restore method textOfValue()

This convenience invokable method was removed in 66ee8985
by mistake, since it's still used in RulesEditor.qml

While at it, optimize arguments to const &

kcmkwin/kwinrules/optionsmodel.cpp

@@ -59,7 +59,7 @@ int OptionsModel::selectedIndex() const
     return m_index;
 }
 
-int OptionsModel::indexOf(QVariant value) const
+int OptionsModel::indexOf(const QVariant &value) const
 {
     for (int index = 0; index < m_data.count(); index++) {
         if (m_data.at(index).value == value) {
@@ -69,6 +69,15 @@ int OptionsModel::indexOf(QVariant value) const
     return -1;
 }
 
+QString OptionsModel::textOfValue(const QVariant &value) const
+{
+    int index = indexOf(value);
+    if (index < 0 || index >= m_data.count()) {
+        return QString();
+    }
+    return m_data.at(index).text;
+}
+
 QVariant OptionsModel::value() const
 {
     if (m_data.isEmpty()) {

kcmkwin/kwinrules/optionsmodel.h

@@ -55,8 +55,8 @@ public:
 
     void updateModelData(const QList<Data> &data);
 
-    Q_INVOKABLE int indexOf(QVariant value) const;
-
+    Q_INVOKABLE int indexOf(const QVariant &value) const;
+    Q_INVOKABLE QString textOfValue(const QVariant &value) const;
     int selectedIndex() const;
 
 signals: