Make Notebook selection tree permanently visible e.g. on the left hand side of the screen

If I may propose another thought for the new Zeppelin GUI, it may be very helpful if the notebook selection tree would not be a drop-down-menu but shown permanently, e.g. on the left border of the screen (some kind of File System Explorer view where you can also show the tree along with the current folder conents). I am often switching back and forth between various notes and this would speed up this process significantly.