Fix legend text not customisable for QgsSimpleLegendNode items

cherry-picked from 4a9a936

python/core/layertree/qgslayertreemodellegendnode.sip

@@ -148,7 +148,7 @@ class QgsSimpleLegendNode : QgsLayerTreeModelLegendNode
 #include <qgslayertreemodellegendnode.h>
 %End
   public:
-    QgsSimpleLegendNode( QgsLayerTreeLayer* nodeLayer, const QString& label, const QIcon& icon = QIcon(), QObject* parent /TransferThis/ = 0 );
+    QgsSimpleLegendNode( QgsLayerTreeLayer* nodeLayer, const QString& label, const QIcon& icon = QIcon(), QObject* parent /TransferThis/ = 0, const QString& key = QString() );
 
     virtual QVariant data( int role ) const;
 };

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -392,19 +392,22 @@ void QgsSymbolV2LegendNode::updateLabel()
 // -------------------------------------------------------------------------
 
 
-QgsSimpleLegendNode::QgsSimpleLegendNode( QgsLayerTreeLayer* nodeLayer, const QString& label, const QIcon& icon, QObject* parent )
+QgsSimpleLegendNode::QgsSimpleLegendNode( QgsLayerTreeLayer* nodeLayer, const QString& label, const QIcon& icon, QObject* parent, const QString& key )
     : QgsLayerTreeModelLegendNode( nodeLayer, parent )
     , mLabel( label )
     , mIcon( icon )
+    , mKey( key )
 {
 }
 
 QVariant QgsSimpleLegendNode::data( int role ) const
 {
   if ( role == Qt::DisplayRole || role == Qt::EditRole )
-    return mLabel;
+    return mUserLabel.isEmpty() ? mLabel : mUserLabel;
   else if ( role == Qt::DecorationRole )
     return mIcon;
+  else if ( role == RuleKeyRole && !mKey.isEmpty() )
+    return mKey;
   else
     return QVariant();
 }

src/core/layertree/qgslayertreemodellegendnode.h

@@ -181,14 +181,15 @@ class CORE_EXPORT QgsSymbolV2LegendNode : public QgsLayerTreeModelLegendNode
 class CORE_EXPORT QgsSimpleLegendNode : public QgsLayerTreeModelLegendNode
 {
   public:
-    QgsSimpleLegendNode( QgsLayerTreeLayer* nodeLayer, const QString& label, const QIcon& icon = QIcon(), QObject* parent = 0 );
+    QgsSimpleLegendNode( QgsLayerTreeLayer* nodeLayer, const QString& label, const QIcon& icon = QIcon(), QObject* parent = 0, const QString& key = QString() );
 
     virtual QVariant data( int role ) const override;
 
   private:
     QString mLabel;
     QString mId;
     QIcon mIcon;
+    QString mKey;
 };