PWA AppIndicator support

[CrimsonFork]

It would be nice if the Progressive Web Apps could use AppIndicators to bring them closer to native desktop integration. This would enable it to e.g. let the user monitor the favicon/badge state and allow for easy opening of the web app regardless of what the user is otherwise doing, even if Floorp isn't focused. Perhaps it could be possible to also keep web apps running in the background even if no browser window is open. This would be most useful for chat applications, but We imagine there's other good uses, too.