represent project configurations more cleverly

Configurations of projects, products, files etc. are currently represented as QVariantMaps (class PropertyMap). There's no data reuse, e.g. Groups that override just one property get a full copy of the product's configuration.

Solve this by creating a configuration representation that

• incorporates the concept of prototype chaining for avoiding full copies
• does not need a full JS evaluation a priori (see evaluateAll in loader.cpp)
• is serializable