Set ESource's connection-status to disconnected on EBackend's dispose
Set ESource's connection-status to disconnected on EBackend's dispose

As long as the ESource is associated with only one EBackend instance,
which it is, then the backend dispose means that the source is not
connected anymore, thus mark it as such. This may allow to not show
credential prompts for sources which are not interested in them anymore.

matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server