Don't ask option for unmounting

I make heavy use of a couple of NFS mounts so the repeated prompts to
unmount them when I close ROX windows have been getting on my nerves.
I've added a "Perform the same action in future for this mount point"
button to the dialog and created a fork called th.git at repo.or.cz.

It would be nice if there was a way to reenable the dialog if you change
your mind later. A logical place would be the mount point's menu, but
that would mean adding an extra menu item that would probably have to be
hidden for other types of icon. Do you think iot's worth doing?

-- 
TH * http://www.realh.co.uk

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