Classify plugins in categories

As we are publishing more and more plugins via our plugin repository, we need to classify them in various well-defined categories so that the user can easily find what he is looking for. This blueprint can be broken down into the following tasks:

 - Define a set of categories;
 - Define how to store the plugin category in the egg's metadata and implement its extraction;
 - For each plugin we currently have/publish, pick the best suited category and update its metadata accordingly;
 - Design and implement by category display in the UI.