scroll up and down buttons with settings

Adds a clean set of up/down/settings buttons. Settings panel includes a hide timer and a transparency toggle.

this is a very helpful userscript that allows you to scroll to the top and bottom of a web page.

Marjanoo edition - where you have the option to make the button transparent (so that the buttons won't get in the way):
https://greasyforks.org/en/scripts/546045-scroll-up-and-down-buttons?version=1664766