Implement a view sorted by keywords

[cboulanger]

Currently, the "keyword" field in Manifest.json plays only an optional, informative role. At the same time, the current view, sorted by owner, and repository, is not helpful for finding useful packages. This could be changed by creating a (at some point default) view that sorts the packages by keywords, thus motivating package authors to provide content for the "keyword" field in the Manifest. This could be supported by a message in the build process of the packageviewer and/or when executing qx publish, warning if the keyword field is empty.