Make named crops start at (0,0) instead of in the middle of the scanned page

When a user wants to crop based on a named crop (e.g. A4, A5, Letter, Legal), this is because the source is (presumably) in that format. Therefore, the probability of the source of the scanned image being nicely aligned to (0,0) is greater than it not being at (0,0) (namely in the middle of the scan). Without this change, a user who wanted to -for instance- crop to A4 (or Letter, or ...) would have to right-click, select Crop -> A4 and then the cropped area would be in the middle of the scan, chopping off the head of the scanned page and including (at the bottom) empty content. This change makes named crops originate from (0,0), therefore saving the user the extra step of having to move the location of the (named) crop to that location. This appears to be a much more natural way for the application to behave. Note that this only applies to named crops.

(This is one of the isolated MRs from Merge Request 26)