This stuff is located in JDom for the rendering
Namespace : html.menu.cpanel
Cpanel menu:
libraries/jdom/html/menu/cpanel.php
and buttons here:
libraries/jdom/html/link/menu/cpanel.php
JDom is gonna disapear in future. All this will be handled with a component helper. Maybe very soon. It could be the next monday, or following.
At the moment, you can override JDom in you component, placing the modified files in "dom" directory of your back-end component.
Just respect the same folders tree, and your custom file will override the native JDom. It is not using the fork system. It was designed before and JDom is independant.
Hope it helps, otherwise, I can give you more details.
If you hold patience, I tell you, this is gonna evolve soon.