Add API for wokring with dso_list element

[jfilak]

As proposed by Matej Habrnal:
https://lists.fedorahosted.org/archives/list/crash-catcher@lists.fedorahosted.org/message/L6P6GKODXGRUO4NOX4UHUA5THKVWCT6T/

Lets begin with definition of the API and then implement it for the current format fo the dso_element element.