Use the crate from the repo and async-std

[hfiguiere]

1. so that we always build with the latest.
2. fix feature so that it actually work.

[A6GibKm]

Unfortunately, it is not possible to build the demo with GNOME Builder due to the same error seen on CI.

[bilelmoussaoui]

The demo is never meant to use the git version of ashpd. Because of two reasons:

• The one stated above by Maximilano. You can't make it work without opening a sandbox hole
• When a new breaking API release of gtk4-rs is around the corner, we switch to use it in ashpd main allowing people to try the latest APIs, if we ever merge something like this PR, I will have to update the demo continuously which something I don't want to.

I know it makes things easier for developing a new portal implementation, by having a very easy way to test it. But you don't really need a GUI for it.

The GUI is targeting distributions and users to debug portals, not writing ones.

[bilelmoussaoui]

A similar patch is now in main