Port extensions app and portal to GTK4

Subject says it all really.

One thing that's worth pointing out is that extensions must port their preference widget to GTK4 once the portal is ported. Keeping GTK3 support would essentially require splitting the portal into three processes (the ui-less portal, plus one prefs process for each of GTK3/4), and I'm not convinced it's worth the effort.

One thing we could do to allow extensions to support multiple GNOME versions more easily would be to look for prefs4.js first. Not entirely convinced by that either though ...