#83 Show preview for Crop operation

[Michael von Glasow]

When cropping a page, figuring out the correct number of pixels is currently a guessing game. It would be easier if there were some visualization of the new page boundaries that gets updated as the user modifies the pixel values in the Crop dialog (e.g. by displaying a dotted line showing the new boundaries, or by reducing the brightness of areas that will be removed).

[Jeffrey Ratcliffe]

There is already some visualisation of the page boundaries. If you switch from the Hand tool to the Rectangular Selection Tool, then the current selection will be visible.