contact-editor: Add destructor for destroying avatar_selector

Hi,

With the inspector, I discovered with hide_on_delete the AvatarSelector was still there even after the Editor was destroyed.

It means after saving the modification on the contact, selecting another and open a new AvatarSelector, a new instance was created and the old one was still there.

I don't know if it's the proper way to handle it, so please don't hesitate to tell me what you think.

Thanks !