Move all logic out of destructors, so it cannot happen any more that
members of a sub-class are accessed from a superclass destructor, when
those members are already dead.

Logic is now in virtual DestroyImpl() methods.

All SwFrms must be deleted with SwFrm::DestroySwFrm().

Change-Id: Icec5b12e12d5a2d955cb5844d7d4f7ac85ab79cd