Handle order of saved plan elements using rdf sequence

[m-markovic]

Currently we are using a custom property af:belongsToRow which keeps the elements in a row they belong to but does not guarantee the same order of steps within each row as they were created in the workflow builder. https://www.w3.org/TR/rdf-schema/#ch_seq could be probably a better solution - i.e. use sequence to document rows and then for each row another nested sequence for steps.