Refactor and improve ScrollbarMargin.create_margin

[ilius]

This PR refactors and improves the readability of scrollbar rendering code.

And also fixes an issue which is commented in the code

    scrollbar_height = min(
        window_height,
        max(1, int(math.ceil(window_height * fraction_visible))),
    )
    # Without math.ceil, the scrollbar will never reach the end.
    # Unless you increase the scrollbar height by one (like it was done
    # before, implicitly) which makes it less accurate, and may also
    # cause a misconception that there are no items after the last visible
    # item when scrollbar reaches the end before the last item is visible
    scrollbar_top = int(window_height * fraction_above)
    scrollbar_bottom = window_height - scrollbar_height - scrollbar_top

If you test it carefully, you realize that scrollbar height was (usually) bigger by one that what should be (for example if menu height is 16 and you have 32 items in menu, the shown scrollbar height is 9 rather than 8).