Prevent SelectionZone from interfering with nested links and buttons

[ThomasMichon]

Overview

Right now, if the user clicks an <a> or <button> within a selectable item in a <SelectionZone>, the selection logic still executes, so the underlying item becomes selected before the default behavior of the child element gets to execute. This can be a problem if changing the selection state reconfigures the elements, or the modal state blocks the next UX events from firing.

To fix this, SelectionZone should not intercept mouseDown or click events dispatched to a child <a>, <button>, or <input> element, unless that element has data-selection-invoke or data-selection-toggle applied.

[ecraig12345]

Looks good to me, but you should probably also have someone more familiar with this code/selection-related issues take a look.