Add StIconColors object, compute in StThemeNode

A new StIconColors object is used to efficiently track the colors
we need to colorize a symbolic icon.
st_theme_node_compute_icon_colors() is added to compute the
StIconColors for a theme node. (Refcounting of StIconColors means
that we'll typically share the colors object of the parent node.)

https://bugzilla.gnome.org/show_bug.cgi?id=633865

src/st/st-theme-node-private.h

@@ -67,6 +67,7 @@ struct _StThemeNode {
   StBorderImage *border_image;
   StShadow *shadow;
   StShadow *text_shadow;
+  StIconColors *icon_colors;
 
   GType element_type;
   char *element_id;