fdo#61135 New feature in Calc: Stepped Lines graphs - first part of code submitted

[Markus Mohrhard]

Hey Eric,

I pused your patch to master and the test file to the test-file repository.

I only moved in a follow-up patch the handling for gnumeric values
from chart2 into xmloff. IMO the handling of file format issues should
be kept in the filter code and not be moved to the chart core code.

Thanks a lot for this great work. I'm looking forward to the UI change.

Regards,
Markus