re-export feature client_system from wayland_backend

[hack3rmann]

This feature is needed by any project that would use Smithay client-toolkit to integrate with anything via FFI, like wgpu for instance.

[ids1024]

wayland-client now offers a system feature that requires wayland-backend/system, so sctk can also just drop the explicit dependency on wayland-backend and use wayland-client/system here. The feature call also just be called system (the fact it is for clients would be implied.)

A feature flag isn't strictly necessary, since a consumer can add an explicit wayland-client or wayland-backend dependency, but seems appropriate given the crate is already re-exported in https://smithay.github.io/client-toolkit/smithay_client_toolkit/reexports/index.html.

[hack3rmann]

1. Removed the explicit dependency on wayland-backend
2. Renamed the feature to system

Appreciate your concerns on the flag, but I tried using wayland-backend as an explicit dependency with client_system being enabled and it didn't work out for some reason -- maybe I did it the wrong way.
Also, it's more clean to have the flag IMHO. It tells that the crate provides raw FFIs indeed

[hack3rmann]

I saw there was 3 failed CI workflows:

1. Formatting issue (my bad, just solved)
2. ci (1.71.0) -- calloop v0.14.4 does need rustc 1.71.1 or newer. The pipeline fetches rust toolchain v1.71.0
3. Cirrus CI:

Not enough compute credits to prioritize tasks!
Failed to start an instance: INVALID_ARGUMENT: Snap images are not supported due to boot stability.

There's another pending pull request that requires issues 2 and 3 to be solved. Issue 3 could only be solved by the maintainer though issue 2 could be solved via pull-request.

Do I need to open issues for 2 and 3?
What would be better: updating CI workflow's rust toolchain's version to 1.71.1 at least, or downgrading calloop to the nearest version that supports building with rustc 1.71.0?

[wash2]

Updating the CI toolchain version would be good. I'll merge this after that and the last issue can be resolved in another PR.