I initially tried to avoid this, but decided it was necessary given the number
of times I had to repeat the same pattern of casting a variable to void*, and
then casting it back in a part of code far distant from the original type.

This changes our preference callback registration functions to match the type
of the callback's closure argument to the actual type of the closure pointer
passed, and then casting it to the type of our generic callback function. This
ensures that the callback function always gets an argument of the type it's
actually expecting without adding any additional runtime memory or
QueryInterface overhead for tracking it.

MozReview-Commit-ID: 9tLKBe10ddP

--HG--
extra : rebase_source : 7524fa8dcd5585f5a31fdeb37d95714f1bb94922