change the structure of _SeafDirentBase

[nguacon01]

Hello, I did some changes in _SeafDirentBase class I think we dont need self.client attribute cause it exists already in self.repo path of return object could be defined by adding '/' + name we need to add some new attributes in this class:

• parent_dir: path of upstream directory
• I added attr 'type' and it must be 'dir' or 'file' instead of using isdir attribute like you did
• I think we need fullpath attribute of object for upload file stubs

Otherwise, I think it's is more useful if we place _get_upload_link in _SeafDirentBase instead if SeafDir. Cause in some situations, I want to request the upload link from a file which i was changing it and then upload it back to seafile server.

I'm sorry if you have some confuses cause English is not my mother tongue. Hit me back and thank for reviewing my code