form: Allow to use a callable to feed additionalFormData

This allow to provide a callable instead of a set of parameters.

As it's buttons, you only need to know this value at the time you click. So i hope this will reduce some processing.

I use it to feed ROIs geometry to form, which are updated dynamically.

I think i am the only one using additionalFormData right now.

(cherry picked from commit 1f6eb88e)

Co-authored-by: Valentin Valls valentin.valls@esrf.fr