small rework of product sets: products, features and product sets

* split the old idea of a "product" into "product", "feature" and "productset"
* make no real difference between user-defined productsets and predefined ones
* move the definition of the products into a separate file, for better overview and discoverability
* put the documentation directly into that separate file instead of hiding it away in some README in some subdirectory noones knows about
* generate a file product_deps.dot with the graph of the dependencies, showing also the build state (in DOT notation, see attached SVG file generated from that)
* more patterns for the ids of the products
* allow to have multiple productsets/products/features mentioned in the -DPRODUCTSET flag

CCMAIL: calligra-devel@kde.org

REVIEW: 118510
Thanks boud, jstaniek, ingwa for review