[composer] Fix missing descriptions for composition data defined buttons on initial composer load

Author: nyalldawson

src/app/composer/qgscompositionwidget.cpp

@@ -112,6 +112,9 @@ QgsCompositionWidget::QgsCompositionWidget( QWidget* parent, QgsComposition* c )
   connect( mPaperOrientationDDBtn, SIGNAL( dataDefinedActivated( bool ) ), this, SLOT( updateDataDefinedProperty( ) ) );
   connect( mPaperOrientationDDBtn, SIGNAL( dataDefinedActivated( bool ) ), mPaperOrientationComboBox, SLOT( setDisabled( bool ) ) );
 
+  //initialize data defined buttons
+  populateDataDefinedButtons();
+
   blockSignals( false );
 }