Fix UI glitches in popovers on Catalina

[ayoy]

Task/Issue URL: https://app.asana.com/0/1177771139624306/1202121324275642/f

Description:
On Catalina, override popovers' top-level views appearance so that the vibrancy effect is removed,
and then subscribe to app appearance updates to keep them in sync with the rest of the app.

Steps to test this PR:

1. Run the app on Catalina. You can use the review build attached to Asana task.
2. If you're still with me, trigger the following popovers and verify that they don't contain transparent areas around labels and buttons:
   1. Autofill
   2. Save Identity/Credentials/Payment Method
   3. Bookmarks
   4. Add bookmark (from address bar)
3. If you like, you can also do this:
   1. SSH to Catalina
   2. While keeping the popover open, trigger the following command to switch between dark and light mode, and verify that the popover UI is correctly updated to the new appearance:

$ osascript -e 'tell app "System Events" to tell appearance preferences to set dark mode to not dark mode'

(you can't do it from the UI on Catalina machine because popover would disappear when losing focus).

Testing checklist:

• Test with Release configuration
• Test proper deallocation of tabs
• Make sure committed submodule changes are desired

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Internal references:

Software Engineering Expectations
Technical Design Template
When ready for review, remember to post the PR in MM

[mallexxx]

LGTM 👍
(I assume non-working on-page autofill is a known issue and out of scope for this PR)