Only autocomplete with directory names when moving a torrent.
Suport for HOME (~) in tab completion.

Also suport it in add_torrent (making it consistent with move_torrent).
Fix to the tab completion.

Supose that in a directory you have 2 subdirectories named foo and foobar.
If you type foo and <TAB>, we should not append os.sep at the end,
since foobar is still a valid option.
Append os.sep to initial path in add torrent dialog.