Qt: preferences: expand root level.

We have only 5 items, and lots of empty space.
Let's save the user 1 click and give him a better overall view
of categories.
(cherry picked from commit b2cfa07f)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/complete_preferences.cpp

@@ -131,6 +131,7 @@ PrefsTree::PrefsTree( intf_thread_t *_p_intf, QWidget *_parent ) :
             current_item->setData( 0, Qt::UserRole,
                                    qVariantFromValue( data ) );
             addTopLevelItem( current_item );
+            expandItem( current_item );
             break;
 
         /* This is a subcategory */