Adds an asynchronous method for filling tiles, which calls a callback
when the process is complete. Currently this is done by listening to the
notify::state property of the tile, but in the future it should be
implemented by the map source subclasses themselves.

This has several advantages:
- It makes it easier to write tests for map sources
- Error handling can be done using the error parameter of the finish
  function, rather than with an error tile source
- It will be much easier to move the memory cache out of the map source
  chain