Supporting changes:
* Remove the preview.previousActiveLink property from the state tree as
it's unnecessary.
Change-Id: I657decf9425a7a9e2b27a798ed60b162569661d8
Supporting changes:
* Add mw.popups.registerChangeListener, which registers a change
listener that will only be called when the state in the store has
changed.
Change-Id: Ibe6934058327c7f02f7d8092e74a667a5a1c600a