[FEATURE] Adds category group

[rrahn]

Allows external configuration systems to boostrap plugins from different
providers into a core plugin for distribution.

[rrahn]

@jpfeuffer @AlexanderFillbrunn can you point me to the tests, so I can add one for the feature. And to give you a scenario. I want to be able to ship in one plugin version sources from multiple vendors. To show that some of the tools are SeqAn and others from external vendors I can add this on a configuration level and highlight it visually.

[AlexanderFillbrunn]

Makes sense. But maybe the default should be for getCategoryGroup to return null, so people do not have to implement it explicitly and only overwrite it if necessary. I am afraid we don't have any tests for this yet, unless @jpfeuffer wrote some.

[rrahn]

It makes sense to make it not abstract. I'll change this.

[jpfeuffer]

There are some rudimentary tests in the com.gkn. ... . test package.