Since parse() is a method of the Gdk.RGBA class, the GdkRGBA pointer
should be the first argument, and the string the second one, to allow a
more natural binding.
https://bugzilla.gnome.org/show_bug.cgi?id=635879
GdkRGBA is a boxed struct similar to GdkColor, with the difference
that it stores alpha information as well, and colors are stored in
[0..1] doubles, in the cairo spirit.
gdk_cairo_set_source_rgba() has been also added to allow easier handling
of this new type.