Shirley background substraction: select the region

[MarcBHahn]

It seems that there is currently no possibility to select the borders for the Shirley background calculation/substraction.
At least the outcome seems not to differ when the "active region" is changed,
It would be handy to have this, since it is necessary for most of the applications.
I'd like to implement this feature, is maybe someone already working on it in a different repository?