Make BrowserAction actually stateless - its responsibility
is just to map the logical H state of a tab (active/inactive/errored,
annotation count) to the badge state.

 * Convert the per-tab state stored in TabState from
   a string into an object so that we can store additional
   state, starting with the badge count.

 * Move the function for fetching the badge's annotation
   count and updating the H tab state into TabState.

   This decouples the presentation of the badge from
   the logic of making network requests and updating
   the model.