Updated workaround for desktop Webkit quirk where clicking a label will ...

...fire two clicks

This fixes issue in iOS Safari as well

To reproduce the issue fixed by this commit set up a page with
 - an element with ng-click
 - a radio button (with hg-model) and associated label
In a quick sequence tap on the element and then on the label.
The radio button will not be checked, unless PREVENT_DURATION has passed