Fixes Unicode Key File Directories (#2120)

* Fixes Unicode Key File Directories

Adds code so that when loading a file it converts to UTF16 first, to
ensure the files can be opened. Code borrowed from FileUtil::Exists.

* Update src/core/crypto/key_manager.cpp

Co-Authored-By: Jungorend <Jungorend@users.noreply.github.com>

* Update src/core/crypto/key_manager.cpp

Co-Authored-By: Jungorend <Jungorend@users.noreply.github.com>

* Using FileUtil instead to be cleaner.

* Update src/core/crypto/key_manager.cpp

Co-Authored-By: Jungorend <Jungorend@users.noreply.github.com>