Added an option for automatically rotating even pages

When doing a duplex batch scan using an ADF, some devices (e.g. the Brother MFC-8880DN) return the documents' backsides upside down. With this option being selected, all even pages of a batch scan are (losslessly) rotated by 180° automatically.

This is only meaningful for duplex scans. I'm not sure if we could also reliably detect those by searching for "Duplex" or "duplex" in the source string, as apparently, there's no SANE property to reliably know if we're duplex scanning and this would only be string matching ...