Here you go:
Most of the functions here will not work(cause this is incomplete), but as far as for what you are asking for, it does work. Aka, you can add toolbars and actions to those toolbars. You do so by going to tools->scripts->ToolBarUI->configure…, then adding a toolbar in the listbox. Then create a top/left item and picking a reaction. Anything labeled default isn’t working. So you have to set open by to left click and then pick an action and update the reaction. By setting up multiple reactions, you can even save space on a toolbar by hooking some things to left click, some things to middle click and some things to right click.
If you plan to build your own plugin, you may wish to use addAction instead of how I do it via addWidget:
Otherwise, you probably want to follow the same setup of setting up the toolbar at runtime, and binding the actions once window is created.
I also suggest the python developer tools to find all the actions if you plan to do your own.