StIcon: add support for GIcon

Add a "gicon" property so that a GIcon can be used instead of an
icon name, while still getting icon recoloring from the theme.
Also include a compatibility wrapper in libshell until GJS has
support for interface static methods.

https://bugzilla.gnome.org/show_bug.cgi?id=622451

src/shell-window-tracker.c

@@ -941,7 +941,7 @@ shell_startup_sequence_create_icon (ShellStartupSequence *sequence, guint size)
 
   themed = g_themed_icon_new (icon_name);
   texture = st_texture_cache_load_gicon (st_texture_cache_get_default (),
-                                         themed, size);
+                                         NULL, themed, size);
   g_object_unref (G_OBJECT (themed));
   return texture;
 }