Introduce XDDF usermodel to share charts implementation between XSLF and XSSF

[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 about DrawingML.

What is left to be done:

• allow for more than one data series on bar, line, scatter and radar charts;
• complete implementation of first chart types, so that they can be used from XSSFChart;
• implement other chart types.

[Alain-Bearez]

I will work out some more changes in order to have more than one data series per chart, in line with what occurs on XSSF.