PopupElement: Allow $autoCloseIgnore to be overridden
PopupElement: Allow $autoCloseIgnore to be overridden

PopupElement needs to ensure that this.$element is auto-close-ignored,
but it can accomplish this by adding it to $autoCloseIgnore
rather than overriding it.

Change-Id: I81dd678024d218f77f761546e72a536f66218986

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui