Allow reordering sidebar tabs using DND
Much easier than what we currently have which is to shuffle stuff until it matches what you want.
Reordering is limited to current sidebar and current MultiTabBar. This simplifies the implementation. This means that you can only reorder tabs within a sidebar. You can't drop a tab in a different sidebar at an exact location. In most cases, the user will not be shuffling around things constantly so I think this will be enough.