feedback for the xlsx import improvements

Hey Maarten,

thanks a lot for the two awesome XLSX import improvements. If possible it
would be nice to have test cases for the two.

For the bug fix this would be  a simple test document that can be used in
one of the automated tests. For the performance improvement it would be a
document showing the problem but not taking too long. The perf documents
are run under callgrind which means they take around 50 to 100 times the
normal import time during the test. So ideally the document already shows
the problem but still opens in less than 30 seconds.

Do have any other XLSX import issues that you are looking into? Don't
hesitate to contact Eike or me if you have any questions. We can be easily
reached by email or through IRC (erack and moggi in the freenode
#libreoffice-dev channel).

Regards,
Markus