gui2/tgame_cache_options: Set busy mouse cursor during cleaning/purging

These are potentially expensive operations depending on the target
medium, the number of files that must be deleted, and the filesystem
implementation. For example, deleting 12,100 files (around 235 MiB)
takes around 0.4 seconds here.

Using the busy mouse cursor makes it look less like Wesnoth has frozen
entirely and more like there's actual work being done.