Make the map provider choice a preference

Not all of the providers seem to work for me (Yahoo Satellite doesn't
appear to give me any data), but for now I'll leave most of them in.
We can later decide to offer only some of them.

It might be more fun to be able to pick the provider directly from the map
widget. But for now I kept this in the preferences which seemed to be a
good place for it.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'map-providers'