Add ToggleEvent source property

This adds a new Element property to ToggleEvent called source. The ToggleEvent's source property is set to the element which invoked the element which the ToggleEvent is being fired on. Sources can be set up as a parameter passed to element.showPopover(), an element with the popovertarget attribute, or an element with the commandfor attribute.

Fixes #9111.

Author: josepharhar