Show width and height of selection

[Limero]

On macOS, when you create a screenshot of a specific area, the current width and height of the area is shown next to the cursor. This allows the user to quickly measure on the screen and get a screenshot with requested dimensions.

A feature similar to this should be implemented to maim either as the default when creating a screenshot of an area (-s, --select) or with a specific option set.

What it looks like on macOS:

[xPMo]

Maim uses slop for selection, this would have to be implemented there.

[bricewge]

The related issue in slop is https://github.com/naelstrof/slop/issues/79. It should already be possible to do it using shaders but the use of characters in them seems really convoluted.