Use wide-string APIs on Windows.

Use wide-string APIs on Windows.

SGPath and simgear::Dir use ‘w’ versions of POSIX APIs on Windows,
and convert UTF-8 SGPath to wide-strings as part of this.

Includes improved unit-tests for this code, with some very basic
tests of creating and iterating files with Unicode characters in
their names.

No user-visible changes should result from this, on any platform; in
particular wide-string support is still incomplete so FlightGear will
not yet work with arbitrary Unicode paths on Windows.