bnc#862510: PPTX import: Properly show data labels in percent format.

Usually, "General" is "0.00" number format, but in this case, when we
want to show percent value, MSO writes that instead of "0%".

Change-Id: I748719765f58e66f9f3fb43c2b527c6823ef6fa1
(cherry picked from commit 5f47e319428a703ea53ce49d166e7628aaa60789)
Reviewed-on: https://gerrit.libreoffice.org/10779
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>