Do not refresh FTP entries on close

This decreases the performance of file copy operations. Especially,
copying many files causes refresh to occur for each file.

Instead, refresh FTP entries manually after file operations.
Check timeout for FTP connection
Initialize _ftpError and _ftpTimedOut
FtpFileEngine: do not close a opened file implicitly

This hides a user's error. And Qt would do it correctly.
FtpFileInfoCache: introduce renameFileInfo()
Introduce PathComp class

QFileInfo uses Qt's file engine at initialization. So introduce this
class to manipulate just path components without those overheads.
move fixUrl() from FileOperation class to PathComp class
Show a working directory when refreshing entries