When studying, the learning count now indicates the number of
learning cards due within the learn ahead limit, instead of the total
number of learning steps required to complete that day.
Also fix the ineffective limit clauses in the learning counts.
We can't preserve the original queues when in preview mode, as
otherwise the due counts report the remaining steps of cards in
the learning queue, instead of just 1.
Rather than the rather complicated approach of making the learning and
deck list code aware of the current mode we're in, preview mode moves
all cards to the review queue when the filtered deck is built - just as
cards are moved to the new queue in Anki 2.0.x. The reason for the
review queue is that users were frequently confused when cards appeared
as new - hopefully this is slightly less confusing.
