Support Table Styles

Excel tables have their own styles, including 60 or so "built-in" styles defined in the Open Document specification.

I'm teaching POI about them for the 3.17 release, and adding some functionality and interfaces to bridge common information between conditional formatting and table styles, since both rely on the same underlying XML element type.

This means an opportunity to refactor my changes suggested in #461 to abstract out the CSS generation further and reuse it for tables as well.

Since a given cell can be formatted incrementally by multiple table style elements, such as both row and column styles together, the spec has a defined order of precedence for resolving conflicts. This works well with CSS too, as long as the order of style classes is similarly defined.

Once I have it working I'll submit a pull request that could cover both this issue and #461, but require the latest POI nightly or beta (if we have a 3.17 beta1 out by then).

Start by reading the changes proposed in #461 and the Apache POI table-style and conditional-formatting APIs described in the issue. Trace how CSS is currently generated and how table-style XML elements are represented; done means built-in table styles are supported and the shared CSS handling resolves combined row and column styles in specification order.