Add option to disable auto-hide scrollbars

Currently, scrollbars automatically hide when not hovered, and there is no option in the GUI to disable this so that they are always shown at their full size.

An option could be added to the Xfce GUI settings to disable this, which would add GTK_OVERLAY_SCROLLING=0 to the /etc/environment file or do something similar.

Additionally, I would disable this option by default, as it would be more suitable given that Xfce is a lightweight desktop environment with few animations, making it a better fit.