ignore viewport scroll events when user touching screen

Experimental change to ignore viewport notifications of size change and/or scroll if the user is touching the screen.

This is the change @AdrianVasiliu suggested, but on testing it I can't see any benefit, so I'm reluctant to check it in.  If you can find a test case where this helps, let me know.

From #310, @AdrianVasiliu wrote:
>  I still think the periodic call of Viewport's checkForResize() is a potential risk for the ability to scroll inside the popup... You can test it yourself by removing the check oldbox != newbox [from Viewport.js] - it does cancel the scrolling in my testing. 

Yes, if I modify Viewport.js that way, I see that problem on delite/tests/functional/popup.html, in the "show tall drop down" dropdown (tested on my iPad).

> There is at least situation which actually scrolls the viewport: when the scroll inside the popup arrives at the bottom or top. 

Yes, I can see that with the "show tall drop down" button on delite/tests/functional/popup.html, but for me it's working with the current code base.