That would not make much sense to a lot of people. Toolbar buttons
typically are left-clicked. If you had to click a combined
forward-backward button with left button to go back and middle-click to
go forwards, that would be tricky to adjust to, and what is the point?
You cannot combine everything. Combining bookmark with reload-stop does
not make sense to me at all. If I want to reload a page or stop it from
loading, will I want to bookmark it as well? Not likely.