[#3950] Ability to perform an attended transfer

sflphone-client-gnome/src/contacts/calltree.c

sflphone-client-gnome/src/dbus/callmanager-introspec.xml

sflphone-client-gnome/src/dbus/dbus.c

sflphone-client-gnome/src/dbus/dbus.h

sflphone-common/src/audio/codecs/g722.cpp

sflphone-common/src/dbus/callmanager-introspec.xml

sflphone-common/src/dbus/callmanager.cpp

sflphone-common/src/dbus/callmanager.h

sflphone-common/src/iax/iaxvoiplink.cpp

sflphone-common/src/iax/iaxvoiplink.h

sflphone-common/src/managerimpl.cpp

sflphone-common/src/managerimpl.h

sflphone-common/src/sip/sipvoiplink.cpp

sflphone-common/src/sip/sipvoiplink.h

sflphone-common/src/voiplink.h