The goal here is to support splitting the sidebar into two panels.

• Each sidebar should be able to split once - we're not going to infinite recursion here
• An unsplit sidebar is split by dragging a tab button to the lower third (?tbd) of the sidebar tab bar.
• When the last tab button of a split sidebar is dragged to another sidebar, the sidebar unsplits
• The sidebar tab bar needs handle not having enough vertical space for all of the tab buttons - collapse to a drop-out menu for the overflow tabs.