Extend XREFScene

This little extension will expose the two functions "GetXRefFileName
index" and "SetXRefFileName index name". The bug which this allows us
to overcome is as follows...
When you load two XRefScenes which are the same file, the command
"XRefs.GetXRefFile i" will only ever return the first one. This is
because the names are somehow used as an ID.
The workaround allows us to rename the XRefScenes so the
"XRefs.GetXRefFile i" will now return the correct reference. It is
necessary to rename the XRefScene back to it's original name before
saving otherwise it will try to load a different or non-existing .max
file as the XRefScene.