parseExternalDocName() also for ConventionXL_A1::parseAnyToken(), fdo#83481

to make 645bccf7a5b91d794d387286deca583ea41f3516 actually work for both,
ExcelA1 and ExcelR1C1 address conventions.

Change-Id: I5392dfb6a8ec6af784957181c722d519921fc065