Limit the initial feature request sent by expression builder widget

Should help avoid short delays in opening the expression widget
on slower layers (such as with the Postgres provider where features
are fetched in a minimum batch of 2000 by default)

src/gui/qgsexpressionbuilderwidget.cpp

@@ -548,7 +548,7 @@ void QgsExpressionBuilderWidget::on_txtExpressionString_textChanged()
     {
       // no feature passed yet, try to get from layer
       QgsFeature f;
-      mLayer->getFeatures().nextFeature( f );
+      mLayer->getFeatures( QgsFeatureRequest().setLimit( 1 ) ).nextFeature( f );
       mExpressionContext.setFeature( f );
     }
   }