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