Allow clearing tile cache

[Carifio24]

The Windows client has functionality to allow clearing the tile cache (triggered via pressing F5). It would be useful to have the same sort of cache-clearing functionality in the web engine, although in this case obviously each client could have a different triggering mechanism. This is something we have an occasional need for in CosmicDS, but I think it would be a generally beneficial addition.

I'm happy to look into how to implement this, but any thoughts are welcome.