Add support for dragging the boundaries between panels

[Keavon]

This should bind an event listener on the document for drag events after a mousedown event on the gutter between tabs (where it's currently set to make the CSS apply a drag arrow icon to the user's cursor). The updated values should be pixel amounts for the flex basis, accounting for the gaps. This task can start as frontend-only, but eventually a follow-up task should save this to the editor backend. A related task should make it possible to drag around and dock panels and tabs.