wayland-scanner can generate header files that only include
wayland-client-core.h using a flag.
Add a core_only option to scan_xml to support this use case.
- Change `scope` kwarg to `public` boolean default to false.
- Change `side` kwarg to `client` and `server` booleans.
- Document returned values
- Aggregate in a single unit test because have lots of small tests
increases CI time.
Fixes: #10040.
Mark Bolhuis