Categorize entries by only one category if tool, framework or game engine

[Trilarion]

And define the order and what takes precedence and define the categories.

Games can have multiple games categories. Tools/Frameworks/... (non-games) can have only one main category but also additional games categories (but do not count as games then).

[Trilarion]

Now done. Still needs some checks and propagation into the statistics, website and markdown toc files output. The term used is now game or non-game.