refactor while we're at it

sflphone-common -> daemon
sflphone-client-gnome -> gnome
sflphone-client-kde -> kde
sflphone-plugins -> plugins

The SIP credentials are transmitted (between client and daemon) and
(un)serialized (in the config file) as a list of items holding:
    - realm
    - username
    - password

The password associated to the SIP account will be removed as it is
redundant with the first item of the list.
idem for the authentication username.

[#2493] Hide volume controls when using pulseaudio and add a signal to the common part to advertise that we've changed the audio manager

We now store interfaces by name