Deprecate / remove DOMUtil

The DomUtil class is unnecessary. It has three functions which are better handled using standard DOM Methods:

Calls to DOMUtil.addClass(el, name) could be replaced with el.classList.add(name)

Calls to DOMUtil.removeClass(el,name) could be replaced with el.classList.remove(name)

Calls to DOMUtil.hasClass(el,name) could be replaced with el.classList.contains(name)

Before today, that left only isInHorizontalViewport(element), which is only used in TimeAxis... so instead of leaving it hanging around, I just moved it.

If I felt like we had a better method of testing UX changes, I might have gone ahead and put in those changes above, at least within DOMUtil but it feels a bit risky.

This issue's title references deprecation, but I don't even know if that's necessary. Does anyone really use anything that deep in the "public" API for TimelineJS?

(This issue inspired by some code inspection around integrating #798)

Start with src/js/dom/DOMUtil.js and inspect its callers, especially TimeAxis, to determine whether the utility is still used or exposed as part of TimelineJS's public API. Trace the class helpers and the moved isInHorizontalViewport usage, then confirm that callers can use standard DOM methods and that the utility can be removed or deprecated without breaking the project.