added fn to prevent multiple GUI updater instances
added fn to prevent multiple GUI updater instances

ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop