With Cage becoming more popular since its mention on Phoronix and
therefore getting more use-cases than just my own project, add XWayland
support. The refactoring of 2cf40f7 makes this much easier. Note that
this is a no-cost addition for those of us not using XWayland as it is a
compile-time option that needs to be explicitly enabled by adding
`-Dxwayland=true` to your meson command.
Since this is inherently output independent, we can move this to here
and avoid the inexistance of an output we ran into in the previous
commit.
Warping the cursor is no problem here either: since we restrict
ourselves to a single output, there won't be any confusing UX by having
the cursor jump from one output to the newly attached one.
This makes Cage much easier to maintain. Not only is it easier where to
look and to maintain a mental model of the code, there is also more
encapsulation, better abstractions and better extendability.
