TpBaseChannel: protect against registering multiple times
TpBaseChannel: protect against registering multiple times

Introduce a boolean value to track whether the object has been registered on
the bus, and ensure that it is false before we attempt to register.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib