Add GdkRGBA struct to supersede GdkColor

Add GdkRGBA struct to supersede GdkColor

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.