Reducers as a whole are a WIP, but this implements a baseline from which
to add more.
Changes:
* Create ext.popups.reducers to house all reducers
* Create reducers for preview and renderer state manipulation
* Create rootReducer by combining preview and renderer reducers
* Add QUnit tests for reducers
* Move action types into ext.popups.actionTypes
* Extract rootReducer from boot.js
Change-Id: I8a2296c6846cd4b0552a485e671af1d974944195
