Be more explicit about what's state in dismissable

Move _activeCardDragEndPoint near build() so it's more obvious that it
is part of the build state.

Make a couple of functions use setState() since they modify variables
that are used by build().

Add a more detailed comment to the empty setState() call, since those
are dubious in general and need explaining when they occur, lest people
start using them as magic incantations to Make Things Work.