Show rectangle selection size in user interface

## Requested feature

When using the rectangle selection tool, it would be great to be able to see the size of the current selection as you are selecting an area.

### Possible drawbacks

n/a

### Possible alternatives

n/a

## Use case

I am a developer and am often looking at screenshots or mock ups. It is often useful to be able to measure a distance between to elements or the size of an element using the selection tool. Once the correct size is selected, then being able to copy and make a new image from the selection.

Start by locating the rectangle selection tool and the interface code that updates the current selection while dragging. Add a visible width and height readout during selection, then verify that it updates as the selected area changes and remains useful for creating a new image from the selection.

Written by the indexing model from the issue text.