* varobj.c (varobj_update): Report changes as vector. Also
return not just a list of varobj, but a list of special structures
that tell what exactly has changed.
* varobj.h (enum varobj_update_error): Rename to
varobj_scope_status.
(struct varobj_update_result_t): New.
(varobj_update): Adjust prototype.
* mi/mi-cmd-var.c: Adjust for changes.