Fix FileName::realPath() on Windows

Fix FileName::realPath() on Windows

The realPath() implementation on Windows works only for files and
not for directories. By using an API available starting from Vista
it is possible to fix it in a simple way.

I also took into account using the Qt QFileInfo::canonicalFilePath(),
but it turns out to not work when a path component is a junction
(tested with Qt 5.14.1).

Due to this, it is not possible compiling or using LyX on Windows
versions earlier than Vista.