`FileDialog::set_directory` needs canonical path on Ubuntu

[Ultraxime]

Hello,

I stumbled upon an issue while trying to set the directory of a FileDialog with Ubuntu.

When you set the directory using a relative path, even if path.exists() is true, the FileDialog ignores it, without a message. But if I canonicalize it before, using std::path::Path::canonicalize, it is working as expected.

I was wondering if it was an intended behavior but was not written in the documentation or an issue.

[Bob620]

This also impacts windows.