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-util.h

@@ -0,0 +1,16 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+#ifndef __SHELL_UTIL_H__
+#define __SHELL_UTIL_H__
+
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+char *shell_util_get_label_for_uri (const char *text_uri);
+GIcon *shell_util_get_icon_for_uri (const char *text_uri);
+GIcon *shell_util_icon_from_string (const char *string, GError **error);
+
+G_END_DECLS
+
+#endif /* __SHELL_UTIL_H__ */