docs: Add recipe about implementing a ClutterEffect

docs: Add recipe about implementing a ClutterEffect

Add a recipe showing how to implement two simple
effects, based on ClutterEffect: an always gray background,
and a border with configurable width and color.

Also explains the necessity to queue a redraw on
the associated actor if the effect's properties change,
and shows how to implement that.

The example gives the GObject code for both effects,
as well as an example application showing how to use them.
The example also demonstrates how to disable/enable an effect,
making the border round an actor togglable.