Stream GetFile() accessor

[troelsk]

This is useful if you need to do the actual reading or writing with code not using wxWidgets

    ::writer_fn(stream.GetFile()->fp());

The stream methods, OnSysRead OnSysWrite, maps directly to the underlying file object so making the file member public is safe.

[troelsk]

GetFile()

[VZ]

(In [73633]) Add GetFile() accessor to wxFileStream classes.

Allow to retrieve the underlying file used by the stream.

Closes #15093.