Fix new backup creation during closing a document

[jeremypw]

Fixes #1343

Rather than risk altering the sequence and timing of events on closing a document this simply adds another flag to the document object to indicate that it is in the process of being closed. This flag stops a new backup being created when trailing whitespace is stripped during closing.

[colinkiama]

As I mentioned in the linked issue, the code works fine however, there may be an bug with stripping trailing whitespace in files with a large amount of lines to deal with in another PR.