adding method to ModifiableItemGroup

[nicolas de loof]

Hi folks,

for JENKINS-14902 I would like Jenkins.createProjectFromXML to be part
of ModifiableItemGroup interface
side effect is that adding such a method to an interface would break
backward compatibility for plugins that implement this interface (even I
don't know such a plugin but cloudbees folders)

option 1 :
use "duck typing" style and call createProjectFromXML by reflexion on
ItemGroup if such a method exists

option 2 :
create a new interface to extend ModifiableItemGroup and
add createProjectFromXML

option 3 : add method to ModifiableItemGroup

wdyt ?