Introduce infrastructure for compositor extensions

The scripting api is not suitable for implementing all features that should not be implemented in libkwin. For example, the krunner integration or screencasting are the things that don't belong to be compiled right into kwin and yet we don't have any other choice.

This change introduces a quick and dirty plugin infrastructure that can be used to implement things such as colord integration, krunner integration, etc.

This merge request is a part of: !436 (merged)