[StThemeNode] Add text-shadow property

Reorganize the existing code which parses the -st-shadow property
to allow parsing different shadow properties and add support for
the text-shadow property.

https://bugzilla.gnome.org/show_bug.cgi?id=624384

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

@@ -47,6 +47,7 @@ struct _StThemeNode {
   char *background_image;
   StBorderImage *border_image;
   StShadow *shadow;
+  StShadow *text_shadow;
 
   GType element_type;
   char *element_id;
@@ -66,6 +67,7 @@ struct _StThemeNode {
   guint foreground_computed : 1;
   guint border_image_computed : 1;
   guint shadow_computed : 1;
+  guint text_shadow_computed : 1;
   guint link_type : 2;
 
   /* Graphics state */