Add virtual destructor for AbstractFileWorker class

If a class has a virtual member function, the class should have a
virtual destructor.
Modify messages more informative
Check the status of open() as well when removing a file
Implement a rename opration for FTP
Add RenameFileWorker class
Remove a cache entry when removing a file
Implement move operation
Disable autoreset of a progress dialog when copying or moving
Abort to access to FTP buffer when an error occurs while transfering
Set source to a new name after renaming
Refresh file info cache after transfering

By this, tricks for FTP refresh can be removed.
Set _urlInfo in case of '/'
Connect to FTP server only if a entry is a directory
Refresh entry view only in case of FTP on removing

QFileSystemModel works fine with a local operation.
Add comments why there is need to refresh entry view
Add to save/load UI settings
Preserve the appearance of entry view after refreshing

However, the current focus position is still not preserved when removing
a file on a FTP server.