altoslib: Fix map preloading callbacks, run in separate thread

The map storage and tile callbacks were muddled together. Create
clearly separate states for map data and have status updates be
delivered when registering for new status events so that registration
is sufficient to track the state without an explicit call to get the
current state.

Run the map tile creation in a separate thread so that even checking
status of files on disk runs out of the UI thread.

These fixes serve to make the pacifier update more smoothly, and also
not over/under count tile loading so that the loading actually
completes when all of the tiles are loaded.

Signed-off-by: Keith Packard <keithp@keithp.com>