FR: Allow clearing of slippymap's tilecache

[Claudius]

This is a feature request to
(1) allow clearing of the slippymap's tilecache
(2) possibly allow selective clearing of only the cached tiles in the current map view.

Use case:
Mapping in a relatively blank area. After two days you still see the cached tiles in the slippymap download window although meanwhile there are mapped features already. Allow to clear or refresh the cached tiles to be able to select the correct mapping area for downloading.