Major Account registration, state reporting, update.

* Renamed setRegister/setUnregister to sendRegister/sendUnregister, reflecting
  more precisely what the function does.
* Renamed (un)registerAccount to (un)registerVoIPLink in account/iaxaccount/sipaccount
* Renamed (un)registerVoIPLink to (un)registerAccount in managerimpl.cpp|h
* Prepared the way for dynamic registration reporting (to the GUI). Now has
  more precise way to describe reg. state: Unregistered, Trying, Error, Registered
* Delete `_link` on SIPAccount/IAXAccount destruction
* All sync of the regist. state with the Manager, is now done in
  VoIPLink::setRegistrationState()
* Document all the way