Share chart data implementation between XSSFChart and XSLFChart through XDDF

[Alain-Bearez]

The main goal of this work is to be able to:

• duplicate a slide that has a chart on it (https://bz.apache.org/bugzilla/show_bug.cgi?id=57835);
• more easily access definitions for axes;
• more easily set data series for axes;
• share code between XSLF and XSSF: XSSFChart and XSLFChart are using XDDFChartData.

What is left to be done:

• complete implementation of first five chart types;
• implement other chart types.

[aperrot42]

A shame I decided to go with docx4j to implement a higher level charting library for pptx... We would have had a lot of common code to share.

[asfgit]

Can one of the admins verify this patch?

[onealj]

Skimming through the changes, looks good to me. I think this is a good idea!

[Alain-Bearez]

@onealj After this pull request will have been merged, I suggest you continue your review on the next PR related to XDDF: #72

The third PR about XDDF is not ready yet.

[onealj]

This PR looks good and ready to merge. We should probably have you sign a contributor license agreement before merging. I sent an email to your cua.li address with details.

[centic9]

CI also is green on commit de37f95, see https://builds.apache.org/view/P/view/POI/job/POI-Github-PullRequests/7/