Define a new Seam invalidation mechanism

Invalidations using @Observers have 2 major drawbacks :

• invalidations are synchronous
• invalidations may be done several times, even if the target component is never used for real

Is some case, it is interesting to reverse the invalidation model : invalidate the component only when it is called.
In order to avoid boring code, most of the invalidation mechanism must be hidden into base classes and interceptors.