Change version of license GPL 2 -> GPL 3

sflphone-gtk/src/accountwindow.c

sflphone-gtk/src/accountwindow.h

sflphone-gtk/src/actions.c

sflphone-gtk/src/calllist.c

sflphone-gtk/src/calllist.h

sflphone-gtk/src/configwindow.c

sflphone-gtk/src/configwindow.h

sflphone-gtk/src/dialpad.c

sflphone-gtk/src/dialpad.h

sflphone-gtk/src/main.c

sflphone-gtk/src/menus.c

sflphone-gtk/src/menus.h

sflphone-gtk/src/screen.c

sflphone-gtk/src/screen.h

sflphone-gtk/src/sliders.c

sflphone-gtk/src/sliders.h

sflphone-gtk/src/statusicon.c

sflphone-gtk/src/statusicon.h

src/account.cpp

src/account.h