eo: Make efl_cast() return NULL if invalid cast

eo: Make efl_cast() return NULL if invalid cast

This makes it work like C++ dynamic_cast<> operator, so that
the return value will be NULL if the object is not an instance
of the given class.

In case of efl_super() we don't do it as efl_super is used A LOT
inside EFL itself (all constructors & destructors, for a start)
and efl_isa is in fact a bit expensive. efl_cast isn't really used
and is intended to be something like dynamic_cast.

For @cedric :)