Rename filesystem::file_name to filesystem::base_name
Remove some unneeded includes

Conflicts:
	src/loadscreen.cpp
Make filesystem.hpp not depend on config.hpp being included first

Conflicts:
	src/filesystem.hpp
Rename file_create_time to file_modified_time, as that's what it does

Conflicts:
	src/savegame.cpp
Remove a comment that is no longer true
Split the parts of filesystem.cpp that don't directly interface with the filesystem off into filesystem_common.cpp

Conflicts:
	src/filesystem.cpp
add filesystem_common.cpp to cmake
Don't try to iterate over things that are not directories