index: support receiving chat state notifications (XEP-0085)

XMPPInterface: send XEP-0115 verification string in presence

XMPPInterface: sendPresence() shortcut method

index: profile window, unregister button

register: cancel button

coffee: move ClientState to tram.coffee

XMPPInterface: unregister(), call to cancel registration

index: progress bar on connect

progress: 0 and 100 are special cases, they need extra visuals

register: progress bar on top