I would like to be able to open multiple sidebar tabs at the same time. Specifically I want to have both the `Properties` and `Filters` tabs open in the right sidebar. I frequently switch between the two, and it would be useful to have both of them on screen at the same time. Neither tab takes up more than half of my window height (and both have collapsible sub-sections which can reduce the space required), so there's plenty of empty space in the sidebar. Further details: *) I suspect that this only makes sense for the right sidebar tabs, as the left ones seem mutually exclusive. *) It would be nice to be able to open more than two tabs -- I rarely use the `Colors` tab, because `Properties` is more important, but having _both_ (along with `Filters`) would be great. *) Ideally the user would be able to change the order of the tabs (probably best implemented in parallel with Bug 491646) *) A drag-able divider between the tabs would increase the usability considerably.