Custom Editor: Selection icon next to folder field

Ahghg... Whenever I get back into having to build a custom editor it feels like I am back to knowing nothing!

I would like to show a field where a user can enter a folder name... it should have a little icon next to it (the circle) that when clicked will open up a folder selection dialog and they can select a folder from there.

So like this:

But instead of loading a script, it only allows the selection of a folder.