Rebrand: part 1

See merge request World/highscore!1

main is now gtk4, so use a stable branch until we migrate.

We were still using swipe groups for collections. Regress the UI a bit,
stop using deck for header bars and use a regular stack instead, so we
can remove swipe groups.

Avoid using a deprecated swipe group.

It's not used by anything else and will likely never be.

It's an error domain with just one error, it doesn't deserve a separate
file.

Just use strings. There's no error handling, no need for hacky properties
on Games and it's overall cleaner. Previously Title was used for lazy
loading, but it's not relevant anymore since loading is threaded.

The replacement isn't a 100% equivalent, but it will be much easier to fix
once title is a string.

There isn't much need to have FilenameTitle when we can also use
GenericTitle and pass a string into it.

We don't have plugins anymore.

Now the plugin types can be easily added there as well.

Just assume a GenericUriGameFactory per platform.

Finally, we only have one uri game factory implementation.