* TileLoader.cpp:

This fixes the most obvious issues with the tile downloading.
The only real other issue left is speed: We need to improve the HttpDownloadManager to have better algorithms to better prioritize the currently needed tiles.

- Fix what is probably Marble's most hated bug:
Tiles being "stuck". The culprit: when a new tile was successfully downloaded and the temporary "replacement" tile was still in the cache, that replacement tile should have gotten removed from the cache to make way for the real one.

* HttpDownloadManager.cpp:

- Increase number of activated downloads to hopefully get the speed a little bit up