Use portals by default

[orowith2os]

It can be safely assumed that a system has portals by default; if they don't, many things will simply fail to work, including, but not limited to, the dark mode preference and many other freedesktop settings.

Those that absolutely refuse to use D-Bus will thus end up with an unusable system in the first place, if they want to use anything desktop-related. So enabling the portal backend by default is fine.

GTK will, of course, fall back to using the portals if it's running inside of a Flatpak environment, but it requires manual workarounds outside of one. And, of course, having a library depend on GTK when it's more or less unnecessary isn't ideal.

Closes (although it's already closed) #99

[PolyMeilex]

Not going to merge this until #66 is resolved (it is mostly implemented already https://github.com/PolyMeilex/rfd/blob/master/src/backend/linux/zenity.rs but needs to be wired in as a fallback).

You'd be surprised how many people actually use those "unusable systems".
Just recently, me merging settings portal code in sctk-adwaita caused startup time problems for some users of winit, because people use portal setups so broken that any calls to them result in ~30s delay (xdg portal simply times out after that period as it can't find or communicate with *-impl portal). So those cases are even worse than people without portal setup whatsoever.

It's supper annoying, but those people exist and there is nothing we can do about that.

[orowith2os]

We can tell them to fix their systems.

[PolyMeilex]

Zenity is now used as a fallback for broken systems, so this is good to go now! 🚀