Stop layer loading spinner when layer is removed

[clydebw]

This fixes a problem where if show_loading is True for a layer, and this layer is removed from the map prior to all tiles being loaded, the loading spinner for this layer continues to spin and is stuck on the map indefinitely.

It is noted that this tends to only be a problem for layers that take a good amount of time to load. Otherwise, under practical circumstances, it is unlikely that a layer remove event would fire before the load event. Therefore, to reproduce the problem described above, you will either need to find a tile layer that is slow to load, or otherwise temporarily disable the bit of code that stops the loading spinner upon the load event.

(Also, this is my first contribution here! If I should be opening an issue or something like that, let me know!)

[martinRenou]

Thanks!