Immediately show progress bar when opening attribute table

...instead of waiting 1 second before first showing it. It's a
placebo which doesn't affect the actual opening time, but
makes it seem more responsive to users by avoiding the
initial 1 second blocking delay before any feedback is
given.

src/gui/attributetable/qgsattributetablemodel.cpp

@@ -422,7 +422,7 @@ void QgsAttributeTableModel::loadLayer()
   {
     ++i;
 
-    if ( t.elapsed() > 1000 )
+    if ( t.elapsed() > 1000 || i == 1 )
     {
       bool cancel = false;
       emit progress( i, cancel );