clutter: add va_marshallers for stage signals

Adds va_marshallers to the ClutterStage signals. This allows for better stack traces to be retrieved when profiling. Additionally, since the generic marshaller was using GBoxedCopy/GBoxedFree functions for the GValue usage, the previous code was acquiring a global reader/writer lock in GObject via g_boxed_free() usage.

With G_SIGNAL_TYPE_STATIC_SCOPE, the generated marshallers can avoid the additional copy/free on the instance.