Use properties in EffectWindow where possible instead of inheritance

If possible use the property to Toplevel or Client instead of the
pure virtual methods. This makes it easier to get a stable ABI.
Adding new methods to the class no longer requires to add a pure
virtual method.

From a performance point of view this change should not matter.
Most EffectWindow methods are not invoked during the repaint chain.
But only in response to an event like a window got added. There the
overhead does not really matter as well the previous implementation
made strong use of dynamic casts which are also not knwon for being
very fast.