Separate packages for wayland variants

[Jonathan Brielmaier]

Happy new year fellow Guix hackers,

I wonder how we usually deal with package variants for wayland support.

In my case I want to make keepassxc working more nicely on sway. It is
as simple as adding `qtwayland-5` to its input list. Increasing its size
from 1271.8 MiB to 1303.5 MiB.

Indicators for a separate wayland package:
* size gets increased noticeable
* "enabling" wayland breaks X.org
* launching for wayland requires certain environment variables (like
icedove-wayland with MOZ_ENABLE_WAYLAND=1)

Indicators for an all-in-one package:
* size increase relative small
* package works with wayland or X.org
* reducing duplicated build runs and space on disk with almost identical
package variants

Any guidance on this topic is appreciated :)

~Jonathan