prefetched / external cacheable web map tiles about monav HOT 3 OPEN

At the moment the internal network manager uses a persistent cache for the 
tiles, but it is limited to 50MB.

Would the following changes achieve what you want to accomplish?:

 - Make the cache size configurable
 - Add "clear cache" option
 - Add option to prefetch tiles along a route

Yes, I noticed QNetworkDiskCache could do the same thing, while the current 
MapnikRenderer's format is yet not capable of doing such seamless caching. Add 
a route to prefetch tiles in specified area and zoom levels would also be 
enough for off line map case.

MapnikRenderer's format would be able to manage adding new tiles etc.. but it 
would not be feasible to delete tiles ( which is necessary unless you want to 
display outdated tiles ). Therefore I will stick to QNetworkDiskCache for the 
moment, although it puts a lot of strain on the file system, i.e., one file per 
tile.