[RF] Make it possible to use parameter titles in `RooAbsPdf::paramOn()`

Closes JIRA issue 6039:
https://sft.its.cern.ch/jira/browse/ROOT-6039

In the issue, it was suggested to make that the default, but I think
this is a too drastic change. People often have long variable titles,
and putting these by default in the plot labels might mess up the plots.

As a compromise, this commit suggests an easy option to change the
behavior of `paramOn()`, just using `T` in the format string instead of
`N`.