Hide total blocked counter for private tabs

Background / User story

We're currently including the number of blocked requests in the total blocked counter, even if those requests have originated from private tabs. Instead, we should not count requests we blocked in private tabs, in order to avoid leaking this information.

Therefore we should hide the total blocked counter in the icon popup for any private tabs. Since the "blocked_total" preference will no longer be updated, we'll also need to find a way to keep the page-specific blocked counter updated.

See eyeo/adblockplus/adblockpluschrome#140 (moved)

What to change

• Design: TBD
• Research: TBD
• Spec: https://gitlab.com/eyeo/specs/spec/-/merge_requests/358
• Legal: https://jira.eyeo.com/browse/ABPUI-46
• Development: Use "stats.listen" message instead of "prefs.listen" to get notified of updates to either blocked counter (see branch).

Hints for testers

• Considering all features will be inactive or hidden, the fallback with the illustration should be shown (refer to the specs here )
• Show fallback for private tabs with non-HTTP(S) sites (i.e. the Settings page or an empty new tab).
• Don't show fallback for:
  • Regular tabs
  • Private tabs with HTTP(S) site
  • Private tabs with HTTP(S) site and Adblock Plus disabled on the page
• The page-specific blocked counter should continue to be updated:
  • The correct value should be shown when opening the popup.
  • The value should update while the popup is open.
• The blocked counter in the title of of the Day1 landing page should display the correct value.

Hints for translators

No translation needed.

Integration notes

• Provide additional messages for the UI to retrieve and listen to changes to the blocked counter:
  • "stats.listen" (with available filters "blocked_per_page" and "blocked_total")
  • "stats.getBlockedPerPage"
  • "stats.getBlockedTotal"
• Provide Stats.blocked_total getter in background page to retrieve the total blocked counter.
• Import skin/icons/slot.svg file.