[gtk,gobject-glue] Cleanup whitespace.
[gtk,gobject-glue] Give meta-marshaller a name.
[gtk,gobject-glue] Let us override the marshaller.

We need to override the marshaller and provide a custom one
for the draw signal in GTK+3 because it passes an object that
is not a <GObject> (<CairoContext>).
[gtk,gobject-glue] Fix ambiguous import.