Add support for multiple sidebars in Nautilus

The sidebar in nautilus is great because it provides very handy features like the directory tree, recent folders, shortcuts, emblemes, and so on. However, switching between the different views in the sidebar is quite annoying. I think it would enhance the usability, if nautilus enables users to show several sidebars at once like it is often done in IDEs (e.g., in Eclipse and so on). With this, we could, for instance, have the tree on the upper left, recent folders on the lower left and emblemes as well as notes on a sidebar on the right side of the main view.