Make it possible to clear map tile caches from the GUI

[M4rtinK]

It should be possible to clear cached map data (which at this time mainly means map tile images) from the GUI.

As this involves deleting folders, it would be preferable if only folders detected as containing map tiles & corresponding to known map layers would be offered for clearing, in case the path to the map folder is set incorrectly or the map folder for some reason contains unrelated data.