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
Fix some number format issues, bnc#862510

Set "LinkNumberFormatToSource" to false, so that format code is not
ignored.
Also, do not inherit format code common for all labels, if there is
specific format code for a data label.

Change-Id: I505311d5df641d61e616e354734bd332609fa122
Add chart import, percentage number formats, unit test.

Change-Id: Id1b73c153f4b09ab94446452e7eb096b529473f4