Refactor common logic into single method

Reduced some code duplication and saved a few bytes (463) by refactoring some
of the basic +addXXX property trackers into a common function.

Replaced:

    _addScrollX()
    _addScrollY()
    _addScrollLeft()
    _addScrollTop()
    _addWidth()

with: 

    _addPropertyTracker(trackerName, propertyName)

Pass in either scrollElement (if specified) or window (if not) to the
tracker during _onScroll.

Changed the tracker's incoming param’s name to el from window as a
better indicator of what might be coming in (though neither is
accurate…)