cubicweb-slickgrid #17045764 Fix clipboardCommandHandler in slick.cellexternalcopymanager.js [in-progress]

cf https://github.com/6pac/SlickGrid/pull/73/

prioritynormal
typebug
done in1.2.0
closed by<not specified>
patch[plugins] Use instance attribute in command.execute() (closes #17045764) [in-progress]