The migration from GdkScreen's monitor API to GdkMonitor left out a way
to get the connector's name of a monitor. While there's no real
guarantee that the connector's name is stable, some system components
used it to uniquely identify a monitor until the next plug in/out event.
Since GTK 3 is API stable, we can only add a private setter and getter
functions pair, without a property.
If we set c_marshaller manually, then g_signal_newv() will not setup a
va_marshaller for us. However, if we provide c_marshaller as NULL, it will
setup both the c_marshaller (to g_cclosure_marshal_VOID__VOID) and
va_marshaller (to g_cclosure_marshal_VOID__VOIDv) for us.
