query builder: make the limit 50 handling more intuitive

[gabor]

currently we automatically add a limit 50 to the sql query being built.
to control this value you have to enable the Order section, and there you can adjust the value.
this is not discoverable enough, we should think about how to make this better for the user.
one approach would be always add the LIMIT widget to the query editor when there is a LIMIT part in the query generated.