Currently the display layout re-parenting logic for 3+ displays resides in JS and is used by the UI when the layout tree structure changes.

We should move this logic into C++ so that it can be shared with the model when a new display is added or the size/resolution changes.

Since we only apply this logic when a drag event completes, any minor lag from the JS/C++ round trip should not be apparent to the user. (The UI will receive a complete update regardless and will recenter and resize the new layout).