[AD] Pluginize the 'Change filename' option

[Eric Botcazou <ebotcazou@xxxxxxxxxx>]

Vincent's recent patch against tools/plugins/datitype.c made me realize that 
the new 'Change filename' (to Relative or Absolute) option should be 
implemented as a plugin.

The only problem was solved by exporting the 'data_file' variable of the 
grabber, which contains the absolute filename of the datafile being worked 
on, hence the big diff against tools/grabber.c.

This will buy us a bit of context-sensitivity for the popup menu: if the 
objects don't have the ORIGIN property, the option doesn't show up.

Applied to mainline.

-- 
Eric Botcazou

Attachment: datfname.zip
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