Testing : data set improvements

In the Testing plugin.
When you create a data set with the output of a transform (Create data set action in the Transform context dialog) the data set is created in the background.
We should ask the user to use the created data set:

"Do you want to use data set <name> on transform <transform>?
[o] Set as Input data set
[ ] Set as Golden data set

[Yes] [No]
"

Also, if we create a new data set with the output, pre-populate the data set name and filename using the transform name..

Also also, if you type in a name of a dataset and the filename is empty, automatically fill in the filename with .csv at the end.