OS-specific code should be separated into different files

[milesrout]

Instead of the maze of #ifdefs in filesystem.cpp, for example, there should be the following:

filesystem.hpp         - the header defining the FilePath class
filesystem.cpp         - the implementation of everything OS-independent
filesystem-win32.cpp   - the implementation of everything Win32-specific
filesystem-unix.cpp    - the implementation of everything that is common to both Linux and macOS
filesystem-macos.cpp   - the implementation of everything macOS-specific
filesystem-linux.cpp   - the implementation of everything Linux-specific

[adam4813]

Is this relevant and needed to get the game done?

[milesrout]

Can't get the game done if the code is a mess