Create a new playground directory for plugins/applets that should not be available by default when we release packages.

This will hopefully make a nice staging area for people developing new applets/services/collections/whatever.

The stuff in playground is not built by default, but can be built and installed from the amarok/playground directory in exactly the same way that amarok is built.

This is my very first go at this, so let me know if I screwed up something or if anything can be done in a smarter way.