[#1883] Add sflphone-client "addParticipant" button for conference

sflphone-client-gnome/src/actions.c

sflphone-client-gnome/src/actions.h

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

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

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

sflphone-client-gnome/src/toolbar.c

sflphone-client-gnome/src/toolbar.h

sflphone-common/src/conference.cpp

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

sflphone-common/src/dbus/callmanager.cpp

sflphone-common/src/dbus/callmanager.h

sflphone-common/src/managerimpl.cpp

sflphone-common/src/managerimpl.h