preferences: remove a lot of bind-and-forget widgets from priv struct
preferences: remove a lot of bind-and-forget widgets from priv struct

Cleans up the code and makes it much less work to add new preferences.

GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets