Remove contact referencing logic from GetContactAttributes

Handles are now always immortal.
Replace Account.AccountPropertyChanged with standard DBus.Properties
Remove legacy Succeeded signal from ChannelRequest
Client.Observer.ObserveChannels -> ObserveChannel
Fix misuse of iterators leading to infinite loop

Hard to believe I actually had to send time tracking this one down...
ChannelDispatchOperation now only handles a single channel
Move ChannelDispatchOperation.handleWith default argument to declaration
Use new Client.Approver.AddDispatchOperation signature