Previously the thread counts shown on tabs in the sidebar were computed from the
full set of annotations in the store, not taking into account any filters that
are active. This was acceptable because the tabs were hidden if a filter was
active. However we are planning to change this and thus we need to compute tab
counts that include the effects of active filters.

 - Change `threadAnnotations` to compute thread counts per tab, as well as the
   thread itself. This is done by not filtering top-level threads in
   `buildThread` but instead doing a filtering pass later, which also computes
   the thread count for each tab.

 - Change `SelectionTabs` to take the tab counts as a prop, instead of
   getting the count from the store.