Add Adwaita to the list of default icon themes

Hardcode the fallback to Adwaita. I'm leaving gnome in here for
now, since we're in the middle of the transition. It will be
removed before 3.14.

gtk/gtkicontheme.c

@@ -1256,6 +1256,7 @@ load_themes (GtkIconTheme *icon_theme)
    * in gnome is a pragmatic solution to prevent missing
    * icons in GTK+ applications when run under, e.g. KDE.
    */
+  insert_theme (icon_theme, "Adwaita");
   insert_theme (icon_theme, "gnome");
   insert_theme (icon_theme, DEFAULT_THEME_NAME);
   priv->themes = g_list_reverse (priv->themes);