Add the ability to set anchors for demonstrating intrapage interactions

In addition to preserving the scroll position, there is another kind of target that would be useful in the prototyping mode: page anchors.

Preserving scroll position might work well if the two pages you are jumping between have an element at exactly the same spot on both pages. However, some wire frame interactions demonstrate intrapage interactions, such as an open/close feature on an accordion menu, or a sub navigation link that jumps you down to a particular spot lower on the page. It would be great if an object - such as a rectangle or a piece of text - could be assigned an anchor so that an interaction could be targeted to jump to that anchor on the specified page. Please 🙂