Manifest3

[AllanWegan]

• [x] Convert to Manifest3 - works until service worker restarts
• [x] Robust ports-based contentscript/background Messaging
• [x] Check/fix all content-to-background communication and background handlers
• [x] Check/fix all background-to-content communication and content handlers
• [x] Make embedded work again
• [x] Merge release branch changes into develop and rebase onto it
• [x] Fix avatar image retrieval not working for extension avatars in embedded
• [x] Find a fix for foregorund tab going to sleep which probably is the reason for random background worker termination
• [x] Fix resetting avatar idleness when background restarts
• [x] Fix resetting GUI visibility when background restarts