Make workspaces rearrangeable from workspace switcher

To rearrange workspaces currently, one would have to take all the windows in one workspace and drag them to another workspace and then drag the windows from the destination to the original workspace. To simplify the process of rearranging workspaces, allow for each space to be dragged and dropped like apps in Plank can be (the same animation could even be used).

A less important feature in the same vein would be allowing applications to be moved between workspaces by making the app icons at the bottom of the workspace switcher draggable (like they were in Luna).