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.