FtpFileEngine: fix to rename a cache entry for a file information
FtpFileEngine: factorize with readDir()
Support SFTP
FileEngineHandler: register each handler in its module

FtpFileEngineHandler in ftpfileengine/ftpfileenginehandler.cpp
SFtpFileEngineHandler in sftpfileengine/sftpfileenginehandler.cpp

This reduces additional works to register each file engine handlers.