#68 Add caching of web maps

[Curt Mills]

Add in caching of web maps/images, so that we can snag
them for
later off-line use. It'd be cool to zoom out to cover
an area, and then have a button that says "make me some
maps". The program itself could write the .geo and the
.gif files for each area, at each
zoom level and put them somewhere useful.

[Curt Mills]

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Caching of all online maps (except for toporama maps or .geo files
containing the "REFRESH" keyword) has been implemented.