falsecolor: Add option to display keys

[norihiro]

Description

This PR adds an option to show a key that shows the color map from 0% to 100% as suggested in #86.

How Has This Been Tested?

Tested on Fedora 39.

General checklist

• [x] The commit is reviewed by yourself.
• [x] The code is tested.
• [x] Document is up to date or not necessary to be changed.
• [x] The commit is compatible with repository's license.