cubicweb-skm #441801 Add FSFile support [open]

Mesh, Commandscript and ExecData should be managed by a FSFile relationship instead of be directly set (use an optional uri attribute and type relation [data|result]).

Note: we currently have redundant is_data/is_result attributes

prioritynormal
typeenhancement
done in0.5.0
load1.000
load left1.000
closed by<not specified>