By default PropertyTracker::evaluate() registers the currently
evaluating binding/tracker as a dependency. This should help with
repeaters and other scenarios where in the run-time we use property
trackers but want to track the overall "dirtyness" in the window with
regards to whether a redraw is needed or not.
The new evaluate_as_dependency_root() function allows skipping this
mechanism and is used for the two trackers in the window.
