[Feature request] Add a simple divider (not the default spacer) as a toolbar item to be used on the sidebars

[cuesta4]

I use the second sidebar for as many things as possible, because it's that good. However, pinning every single item on the sidebar causes it to be a bit of a mess. Having a simple divider, which is not the default spacer (which looks awful and doesn't do the job) would allow for a more organized workspace.