Adding "path" API to get directory of the downloaded files

[XiaoGuo, Liu]

The path API is useful for managing and maintaining all of the downloaded files when using the Download manager. Currently, all of the downloaded files are saved in the space of Download manager, the path is unknown to the client app which uses it though "finished" signal can have it. However, for future access of the downloaded files, there is no way to get the path to manage the downloaded files if there is no on-going downloads.