clapper: Port "Features" to "Enhancers"

[Rafostar]

ClapperFeature(s) objects and how they work (their bus) can be ported to be libpeas plugins (enhancers). They already have a dedicated thread and base class for implementing more of them. By making them into plugins, more can be easily written to suit different needs, there would not be a C programming language requirement anymore and we can go down with dependencies (like libmicrodns for server).

As long as this work does not break current implementation, this can be done already with features simply being deprecated. Otherwise this is gonna be 1.0 material.