Experimental Automatic Tile Downloading

This still has got some issues, that need to be resolved:
- Directories of levels that don't exist yet shouldn't be created if there aren't any valid files to download.
- We need to deal with expired/aborted tile download jobs.
- Fetch the server list from a tiny file from edu.kde.org/marble/server.txt and get the data elsewhere.
- the scale up algorithm seems to have an issue at high zoomlevels. I'll look into this shortly.

And most important:
- provide more data ( Right now it's just a small version of Blue Marble as a proof of concept )