Excel Chart support

[apjanke]

• Convert Java interface wrappers to fully abstract classes, with only the concrete subclasses defining the j property. I think.
• The XSSF-specific classes for things you can create from an XSSFDrawing.

[apjanke]

Added some object wrappers to the code today, covering many of the Chart-related stuff at the ss.usermodel level.

[apjanke]

Note to self: the Javadoc for the XSSF part of POI 3.17 is not readily Googleable on the web. You need to download the poi-ooxml-3.17-javadoc.jar file from Maven Central and extract it locally to get a copy of the Javadoc. That's nice to have locally anyway, since the POI website is sometimes slow.

[apjanke]

TODO next: all the XSSF-specific classes for things you can create from an XSSFDrawing.

[apjanke]

Added some more Drawing and Charting stuff. I think I have wrapper classes for all the XSSF stuff in now.

Now just to test and document it. ;)