Avoid some unnecessary composer legend updates

(cherry-picked from aaa654f)

src/app/composer/qgscomposerlegendwidget.cpp

@@ -884,6 +884,9 @@ void QgsComposerLegendWidget::blockAllSignals( bool b )
   mRasterBorderGroupBox->blockSignals( b );
   mRasterBorderColorButton->blockSignals( b );
   mRasterBorderWidthSpinBox->blockSignals( b );
+  mWmsLegendWidthSpinBox->blockSignals( b );
+  mWmsLegendHeightSpinBox->blockSignals( b );
+  mTitleSpaceBottomSpinBox->blockSignals( b );
 }
 
 void QgsComposerLegendWidget::selectedChanged( const QModelIndex & current, const QModelIndex & previous )

src/core/composer/qgscomposerlegend.cpp

@@ -298,7 +298,9 @@ void QgsComposerLegend::synchronizeWithModel()
 void QgsComposerLegend::updateLegend()
 {
   // take layer list from map renderer (to have legend order)
+  mLegendModel.blockSignals( true );
   mLegendModel.setLayerSet( mComposition ? mComposition->mapSettings().layers() : QStringList() );
+  mLegendModel.blockSignals( false );
   adjustBoxSize();
   updateItem();
 }