It would only 'add' accounts before (even when we clicked on an existing one
to edit it).

It will register if it wasn't, and was now set to 'Enabled',

It will unregister if it was already registered, and we switched to 'Disabled'

Faudra remplir ça les amis!

Now, there should be only an 'Enabled' toggle for each account. It determines
if the account is registered on start-up AND if it's enabled, generally.

TODO: We'll need another parameter, to select which account is the one used
to do calls. Anyways, it wasn't very convenient to use the 'First one enabled',
since you'll often want multiple accounts enabled, and be able to select which
account to use for an outgoing call.

This commit updates the SFLphone daemon and the Gtk+ client.

It removes a whole bunch of useless variables (_state, _shouldInitOnStart,
_shouldRegisterOnStart, _registered). Now the registration state is fetched
directly from the VoIPLink (when called from the Account). So, no more
duplication of statuses.

TODO: SFLphone-gtk will need to support the TRYING and ERROR messages as
'Status' reply.

* 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