Previously, the manufacturer property of the GdkMonitor was NULL,
and having at least PNP id at GdkMonitor.manufacturer makes it
possible to distinguish between different monitors programmatically.
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.
