Turn all applications into application bundles

Point in case:
From one day to the next, Firefox stopped working because I had installed some other applications using `pkg install`, which had seemingly upgraded some system libraries, resulting in the old Firefox version no longer working properly.

Hence I was factually forced to update Firefox as well.

As a result, meet.jit.si no longer works in Firefox, now giving this:


`about:config`, `network.http.spdy.enforce-tls-profile` makes Firefox crash.

It had been working before.

There is no obvious way to downgrade Firefox. (Correction: actually there is the `firefox-esr` package, but that's only the case for very, very few apps.)

**This is exactly what makes me dislike the concept of traditional package managers for managing graphical user-facing applicatiions.**

For this exact reason we need to come up with a system that turns each application into an [Application Bundle](https://hellosystem.github.io/docs/developer/application-bundles) and makes it possible to:
* Get new applications without changing the rest of the system, especially without affecting other applications
* Keep multiple versions of the same applications around in case applications suddenly change in unwanted ways (example given above)

How to do this best, in a way that works for most already-ported applications without too much manual effort?

cc @mszoek