Support for the Plustek OpticSlim 2600 (GT68XX)

Hi,

Last December, I got the Plustek OpticSlim 2600 working properly by adding it to the GT68XX backend. I tested DPIs from 300 to 2400, and they all worked. However, calibration doesn't work and the only way to get the scanner to output anything other than a black sheet was to disable it altogether using the GT68XX_FLAG_NO_CALIBRATE flag. The development was done on the mailing list. I do not currently have access to the machine I developed on and my modified source isn't stored online, so I can't add it to the issue yet, but I will try to do that ASAP.

The OpticSlim 2600 turned out to be very similar to the already supported OpticSlim 2400, as they even use the same firmware, so all that was really required to do is creating a new scanner descriptor struct and updating fields about scanner bed size, etc. I would be happy to be assigned to this issue and help finish the implementation.