Fix bug with element inside anchor

[hdodov]

This PR fixes an issue where the browser follows a link after a drag. The previous code checked if the element emitting the mousemove event is an anchor, but the browser will redirect even if one of its parents is an anchor. Also, the check is moved to the mousedown handler for performance. There's no need to check the DOM for anchor tags on each mousemove event. It's unlikely that the result will change since the last mousedown.

Edit: Also, preventDefault() is called only when the item is not dragged beyond the required threshold. It often happens that you accidentally move the mouse 1-2 pixels when clicking. This would prevent the redirection from happening, when the intention was the opposite. Now, the redirection will be avoided only when you've dragged enough, I.E. you actually tried to drag, not click.