Improve the color swatch accessible to set a proper role
depending on whether the swatch is selectable or not, and
set the checked atk state when appropriate.
https://bugzilla.gnome.org/show_bug.cgi?id=721048
And update the colorchooser a11y test to a) use GtkColorChooser
instead of the deprecated GtkColorSelection and b) match this
change. Pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=721045