Deprecate old shell integration (client-side)

Xdg-shell has been released as stable.

We currently maintain support for:

• ivi-shell
• wl-shell
• xdg-shell unstable v5
• xdg-shell unstable v6
• xdg-shell stable
• fullscreen-shell-unstable-v1

It's a significant maintenance cost to keep all those shells compiling, tested, and fix shell-specific bugs.

We should start deprecating the old desktop shell integrations (wl-shell, xdg-shell unstable versions) so they can eventually be removed when compositor support for xdg-shell stable is good enough.