Build: Use meson's wayland module
Meson gained a wayland module, which allows us to cut out a lot of boilerplate.
I stumbled upon the module yesterday when browsing the meson documentation.
It looks neat, but we should probably wait until meson 0.62 is + more common, so we can depend on it.
There's also the issue that the module is unstable (may change or be removed again), but then I suspect that mutter using is would help stabilizing the module.