Add option to change sidebar double click behaviour

[dnred]

Right now, when double clicking on the sidebar, it opens a new tab. Browsers with normal horizontal tabs have a different behaviour - when double clicking, it closes out of fullscreen. On Windows, there is the top bar that shows up but it's too slow compared to normal browsers with that enabled all the time and when the browser is windowed it requires too much precision to make it fullscreen again. So I suggest giving us an option to change the behaviour of the double click on the sidebar to close out of fullscreen just like on the top bar.

[bcrtvkcs]

I need that option too. Thank you @zen-browser