[Vala] Use alpha in GtkColorButton in GTK3 with Vala

[Satyajit Sahoo <satyajit happy gmail com>]

I want to use an alpha value in the color buttons in my app -
https://github.com/satya164/elegance-colors

Currently I'm using colorbutton.set_use_alpha (true);, as documented in
http://www.valadoc.org/#!api=gtk+-3.0/Gtk.ColorButton

But, while compiling, I get warnings saying,

/home/satya/Workspace/elegance-colors/elegance-colors-prefs.vala.c:4706:2:
warning: ‘gtk_color_button_set_use_alpha’ is deprecated (declared at
/usr/include/gtk-3.0/gtk/gtkcolorbutton.h:95): Use
'gtk_color_chooser_set_use_alpha' instead [-Wdeprecated-declarations]

The code samples in the valadoc website are outdated and the sample on
gnome's website doesn't use alpha. Can anyone help me fix the warning
messages?

I'm using GTK 3.8 and Vala 0.20.1.
-- 
Satyajit Sahoo
Profile - Facebook <http://facebook.com/satya164>,
Google+<http://plus.google.com/u/0/114564489020097923600>
Artwork - DeviantArt <http://satya164.deviantart.com>