Ubuntu and Unity still depend on metacity(!) for this kind of action. So you would find all possible 'titlebar action' possibilities in gconf-editor >apps>metacity>general. Unfortunatelly (as in gnome3) there is no 'window close' option.
As there is no 'out of the box' solution for your idea, my approach would be different.
Most of your application will have a 'close' or 'quit' keyboard shortcut. That might be >ctrl< q for QUIT, or >ctrl< w for WINDOW CLOSE, or >ctrl<>shift< q to QUIT a gnome-terminal.
I would now install the software easystroke and add a mouse gesture for the given command. The same mouse gesture can have a different command, following the active window. Let's say a 'right click' and drawing a 'circle' would mean >ctrl< w for the filemanager, but >ctrl<>shift< q for the gnome-terminal.
This solution would give you a similar ease of use, as 'middle-clicking' the titlebar. Needless to say that I am using this kind of solution... ;)
Here is some video I found on the youtube... easystroke in action