Fix setting symbol for single symbol renderer via legend dbl click

Eg if diagram legend is also shown for layer it's possible for the
single symbol renderer to show a legend item.

(cherry-picked from 5612936)

python/core/symbology-ng/qgssinglesymbolrendererv2.sip

@@ -65,6 +65,8 @@ class QgsSingleSymbolRendererV2 : QgsFeatureRendererV2
 
     virtual QSet< QString > legendKeysForFeature( QgsFeature& feature, QgsRenderContext& context );
 
+    virtual void setLegendSymbolItem( const QString& key, QgsSymbolV2* symbol /Transfer/ );
+
     //! creates a QgsSingleSymbolRendererV2 from an existing renderer.
     //! @note added in 2.5
     //! @returns a new renderer if the conversion was possible, otherwise 0.

src/core/symbology-ng/qgssinglesymbolrendererv2.cpp

@@ -450,6 +450,12 @@ QSet< QString > QgsSingleSymbolRendererV2::legendKeysForFeature( QgsFeature& fea
   return QSet< QString >() << QString();
 }
 
+void QgsSingleSymbolRendererV2::setLegendSymbolItem( const QString& key, QgsSymbolV2* symbol )
+{
+  Q_UNUSED( key );
+  setSymbol( symbol );
+}
+
 QgsSingleSymbolRendererV2* QgsSingleSymbolRendererV2::convertFromRenderer( const QgsFeatureRendererV2 *renderer )
 {
   QgsSingleSymbolRendererV2* r = nullptr;

src/core/symbology-ng/qgssinglesymbolrendererv2.h

@@ -86,6 +86,8 @@ class CORE_EXPORT QgsSingleSymbolRendererV2 : public QgsFeatureRendererV2
 
     virtual QSet< QString > legendKeysForFeature( QgsFeature& feature, QgsRenderContext& context ) override;
 
+    virtual void setLegendSymbolItem( const QString& key, QgsSymbolV2* symbol ) override;
+
     //! creates a QgsSingleSymbolRendererV2 from an existing renderer.
     //! @note added in 2.5
     //! @returns a new renderer if the conversion was possible, otherwise 0.