Support for Manifest V3

[AlexanderS]

The extension should support Manifest V3 because Chrome actively stated to block Manifest V2 extensions.

Possible the only required change is the migration of the background page to a service worker.

[bashgeek]

It actually requires quite a bit of refactoring to make this work properly with the whole feature set - and also still work in both browsers, Chrome and Firefox.

I'll try to have a deeper look into this around the end of the year or Q1 of next year.