Set initial effect selection in layer effects widget to

first enabled effect

Hopefully makes the widget a bit more user friendly - otherwise
the initial settings shown may be for a disabled effect and the
users is left wondering why changes have no impact

(cherry-picked from f67a48a)

src/gui/effects/qgseffectstackpropertieswidget.cpp

@@ -129,8 +129,18 @@ QgsEffectStackPropertiesWidget::QgsEffectStackPropertiesWidget( QgsEffectStack *
 
   updateUi();
 
-  // set effect as active item in the tree
-  QModelIndex newIndex = mEffectsList->model()->index( 0, 0 );
+  // set first selected effect as active item in the tree
+  int initialRow = 0;
+  for ( int i = 0; i < stack->count(); ++i )
+  {
+    // list shows effects in opposite order to stack
+    if ( stack->effect( stack->count() - i - 1 )->enabled() )
+    {
+      initialRow = i;
+      break;
+    }
+  }
+  QModelIndex newIndex = mEffectsList->model()->index( initialRow, 0 );
   mEffectsList->setCurrentIndex( newIndex );
 }