Make some crop parameters editable

[StyXman]

Most of the time I'm cropping a photo, I'm either cutting height or width by half, or cropping at 3/4ths x 3/4ths, so I know the end values. I used to use gwenviews crop tool, which allows me to enter those values by hand, and the the mouse to position the 'cage' in the photo. I would be nice if cropgui would at least allow to edit hose (height and width), altho gwenview also allows editing top and left. An image is worth a thousand words....