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Disallow live selection feathering on XP

Performance is abysmal and not easily fixed, and the new Select ->
Feathering menu provides a fine alternative.

Live feathering up to 100px is still available in Vista/7, though
performance remains poor in Vista and I have no intention of fixing it.
(Vista is effectively "not officially supported" at this point.)
Select -> Feathering menu now implemented

Radius up to 500px allowed; higher radii could be enabled, but at some
point performance becomes unbearable.  500 seems reasonable enough for
now.