macOS: Split drag to create new tabs #10092
Description
Follow-up to #10090
We should be able to drag a split into the titlebar or tab bar to create a new tab.
I actually attempted to do this as part of #10090 and even got it partially working but abandoned it due to the complexity it introduced into an already-complex PR. The approach I took was to introduce titlebar and tab accessory views that only show up while a surface is being dragged and allow dropping that surface onto those zones to create the new tab/window.
I opted for drop zones like that because I wasn't able to reliably make the whole area a drop zone and show visual feedback, since it required some nasty view surgery on the titlebar area.
I also wonder if we expose a draggable surface as a proper UTI type if it'll "just work" with the native macOS tabbing system, but I don't know what that type is or what it expects, yet!