Added FTP support. (List and Download)

Known issues:
1- After canceling download and hiding progressbar, downloaditem's height is not updated.
2- When click on a file I force it to download file (see this comment: [ftpschemehandler.cpp#L194](https://github.com/srazi/qupzilla/blob/Ftp_Support/src/lib/network/schemehandlers/ftpschemehandler.cpp#L194))

Qt5:
It doesn't detect HeaderLocation url with ftp scheme!