pre_paint() and post_paint() implementations don't need
to check whether an effect is disabled: Clutter will
not apply an effect unless it is enabled.
So remove code which checks whether the effect is
enabled or disabled from the example applications and the
documentation.
Add example of a simple background color effect applied via
pre_paint() implementation in a ClutterEffect subclass.
This is a simple effect with an incomplete GObject
implementation (no properties, setters or getters)
to make it as easy to follow as possible.