Option to split the sidebar vertically (i.e. multiple sidebars)

I think it would be very useful to be able to split the sidebar not only horizontally but also vertically, i.e. you'd get to have two sidebars. Additionally, now that there can be multiple sidebars, it would be nice if each sidebar could be docked both on the left and on the right of the main window.

The rationale for this is that i would like to have a full column of bookmarks and a full column with the class view, instead of having to clog these together with other stuff such as e.g. "projects" in one single vertical column.

screenshot_mockup.jpg