add functionality to change units of property values.

Webflow is a good example of this.  Here's a screen shot of Webflow's property editor.

User can click on "PX" or "VW" and change it to a handful of options.  "VW" stands for "Visual Width" here, which is incredibly powerful considering you can say "I want this container to take up 50% of the visual width of this screen"  rather than plug in the pixel size.

Here are the unit options they allow users to change to.