Add Tray icon under a settings button

Motivation: I want it to stay open so I remember to add notes when necessary, but I don't want it to clutter my window list.

It's off by default as per the new HIG.

Looks like this:

(ignore the window title, this is because it's developed in a container)

I'd like it to show up below the header bar, but I couldn't figure out how.

There's a bug where once the tray is enabled, the tray menu options appear at position 0x0 of the screen, but it only happens on Plasma 5.