There is no "Ubuntu source code". Ubuntu is a collection of programs configured in a certain way. You need to identify the individual program that you want to change.
You don't need to create a new operating system to do this, in fact, what you want to modify has nothing at all to do with Linux, and everything to do with your desktop environment (the default on Ubuntu is Unity).
You may or may not need to modify the Unity source to implement what you want (it seems likely that you wouldn't have to, anyway), as I don't know Unity well enough to say. Whether you have to modify the Unity source or not, your answer is not in the Linux sources as they do not handle what you're wanting to modify.