[#2281] Set the useragent per account in the GUI

sflphone-client-gnome/src/config/accountconfigdialog.c

...-client-gnome/src/dbus/configurationmanager-introspec.xml

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

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

sflphone-client-gnome/src/sflphone_const.h

sflphone-common/src/account.h

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

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

sflphone-common/src/dbus/configurationmanager.cpp

sflphone-common/src/managerimpl.cpp

sflphone-common/src/sip/sipvoiplink.cpp

sflphone-common/src/sip/sipvoiplink.h

sflphone-common/src/user_cfg.h