Rename filesystem::file_name to filesystem::base_name
Add filesystem::delete_file

code taken from
https://github.com/AI0867/wesnoth/commit/342ee266cfe02d44488d62a0f6c7c2c23ca86c98
Replace some usage of remove (3) with filesystem::delete_file

https://github.com/AI0867/wesnoth/commit/07bb14bd08134297064502abdf968dc2bfcbb50a