Add appInfo.get_desktop_file_path method

Add shell_app_info_get_desktop_file_path, matching *_get_executable,
*_get_id, etc. This is useful for Zeitgeist.

src/shell-app-system.c

@@ -622,6 +622,12 @@ shell_app_info_get_executable (ShellAppInfo *info)
   return gmenu_tree_entry_get_exec ((GMenuTreeEntry*)info);
 }
 
+const char *
+shell_app_info_get_desktop_file_path (ShellAppInfo *info)
+{
+  return gmenu_tree_entry_get_desktop_file_path ((GMenuTreeEntry*)info);
+}
+
 GIcon *
 shell_app_info_get_icon (ShellAppInfo *info)
 {