[mv3] Support popup windows - via message channels

[emaxx-google]

This adds support of manifest v3 to the "popup-window" helpers. This is achieved by reimplementing the popup==>background page communication using message ports.

Previously, we used two other mechanisms for this communication:

• in the Chrome App build mode we injected callbacks into the popup's global state using the chrome.app.window.create() callback.
• in the Chrome manifest v2 Extension build mode we used "setSelfAsOpener" to let the popup call global functions in the opener page itself.

Neither works in manifest v3 Extensions, hence this commit implements a new approach that should work in all build modes.

[emaxx-google]

@AleksMat This was already reviewed on #1167, but I screwed up merging and it ended up in a branch instead of main. Apologies :) Asking for a re-stamp.

[github-actions[bot]]

Continuous Integration: All tests passed. C/C++ test coverage: 73.59% lines.

[emaxx-google]

Thank you again :)