Provide a way for a user to know the number of pixels while inspecting an image

[virtuald]

Some operations ask for things like min/max widths/heights. However, without an idea of how many units are being shown in an image, it's annoying to guess. There are several ways to provide this data:

• Show a ruler along the sides of the image indicating the number of pixels
• Provide a 'measuring tool'
• When there's a step that's filtering by something, you could display statistics like "min contour width shown is X, max is Y"... etc

I imagine that the ruler would be easy enough to do.

[JLLeitschuh]

This sounds like a reasonable feature.
I'd like to try to make sure we don't bloat the UI with this.
If someone wants to throw up a mock up image of how this would look we should vett this first.