core: Allow retrieving the theme variant via core_get()

To associate frames with the correct style variant, the UI will
need access to the window's theme variant property.

https://bugzilla.gnome.org/show_bug.cgi?id=645355

src/core/core.c

@@ -146,6 +146,9 @@ meta_core_get (Display *xdisplay,
       case META_CORE_GET_FRAME_HEIGHT:
         *((gint*)answer) = window->frame->rect.height;
         break;
+      case META_CORE_GET_THEME_VARIANT:
+        *((char**)answer) = window->gtk_theme_variant;
+        break;
       case META_CORE_GET_SCREEN_WIDTH:
         *((gint*)answer) = window->screen->rect.width;
         break;

src/core/core.h

@@ -48,6 +48,7 @@ typedef enum
   META_CORE_GET_FRAME_Y,
   META_CORE_GET_FRAME_WIDTH,
   META_CORE_GET_FRAME_HEIGHT,
+  META_CORE_GET_THEME_VARIANT,
   META_CORE_GET_SCREEN_WIDTH,
   META_CORE_GET_SCREEN_HEIGHT,
 } MetaCoreGetType;