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.h

@@ -25,6 +25,7 @@
 
 #include <clutter/clutter.h>
 #include "st-border-image.h"
+#include "st-icon-colors.h"
 #include "st-shadow.h"
 
 G_BEGIN_DECLS
@@ -197,6 +198,8 @@ StBorderImage *st_theme_node_get_border_image (StThemeNode *node);
 StShadow      *st_theme_node_get_shadow       (StThemeNode *node);
 StShadow      *st_theme_node_get_text_shadow  (StThemeNode *node);
 
+StIconColors  *st_theme_node_get_icon_colors  (StThemeNode *node);
+
 /* Helpers for get_preferred_width()/get_preferred_height() ClutterActor vfuncs */
 void st_theme_node_adjust_for_height       (StThemeNode  *node,
                                             float        *for_height);