Support dynamic placeholders in "Arg and Vars" utility

[jneug]

It would be nice to have an easy way to access the clipboard in a workflow. It's not that hard using script actions, but it would be nice to have an utility for that. I would suggest to add dynamic placeholders to the "Args and Vars" utility. This way we could not only access the clipboard with "{clipboard}" but also combine it with the current query or easily use date objects.