Restructure the filters store module with the following goals in mind:

* Simplify the structure of user-focus mode
* Make the filtering structure for user-focus filters consistent with
  other filters
* Add some actions and selectors for setting and getting filters,
  clearing the way to start adding some additional filtering
* Be explicit about filter precedence

Only one change here that affects the store's API and that is adjusted
for in `useRootThread`.