webui: widget initialization
- used `ctor_init` instead of `init` to avoid name collision with
existing logic
- `ctor_init` is called right after widget instantiation. Basically support
better inheritance for the old class system which doesn't have proper
contructors
https://fedorahosted.org/freeipa/ticket/4402
Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>