macOS White Bottom Bar, seen in Mail

[HerQ]

Hello.

I've searched a lot but never found what I searched for. I want the nice white bottom bar seen in Mail, Automator, Coda, Color Schemer Studio and so on. An image for making it more clear:

I came across some examples but when trying to implement these I experienced some problems. This one actually is very nice but the NSImage at the bottom is nowhere in the project and I can't find it anywhere. How did he do that? Is there a simple way to create a bar like this?

I've been searching for this a long time with various keywords, no results at all.

Thanks in advance!