FileManager fixes for Windows.

[nutzboi]

Commit 9d8cecc makes file_manager.cpp detect compiling for Windows and compile the correct implementations. This fixes a bug (introduced in the command-manager fork) when updating existing addons using unofficial links on Windows.

Commit f8ed20e fixes linked directory issues by replacing MoveFileExW with std::filesystem::copy and ::remove_all (notice the #include <filesystem>) to allow the filemanager to follow NTFS Junctions (microsoftese for "hard links"), which are for some reason detected as normal directories by std::filesystem::status and due to microsoft's incompetence are almost completely impossible to use or resolve with the Win32 API functions at least with normal user privileges.
This fix is C++17 and later only and is thus incompatible with upstream STK 1.x and 2.x.

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