Add a cropping function

[raacampbell]

There should be a widget with a button that allows the user to crop the data. It should work as:

• User adds a shape layer with a rectangle.
• If the shape layer contains a single rectangle, that enables a "CROP" button in the widget. The button will only be enabled if all loaded stacks are the same size to begin with.
• User clicking the CROP button brings up a confirmation dialog.
• If users asks to proceed, all loaded stacks are cropped and the original data are replaced or a new copy made (the user should choose but "replaced" is probably the best option.

After the cropping, we should probably load the cropped stacks.