Draft: setup-desktop: add sway

It would be great if setup-desktop would support sway as well. This is a draft and meant to spark a little bit of conversation, e.g. if this is wanted, what scope it should have, and where parts of the new functionality would go best.

Setting up the community repositories is, for now, just a copy-paste of setup-xorg-base, but should probably be factored out into another script, either a new or an existing one.

Most of what is in the first draft is directly taken from the wiki:

• https://wiki.alpinelinux.org/wiki/Sway
• https://wiki.alpinelinux.org/wiki/Wayland
• https://wiki.alpinelinux.org/wiki/Elogind

The commit currently has a few TODO's. I'm not sure how to progress with most of these as I'm unfamiliar with the general philosophy of setup-desktop specifically.

Are there any thoughts on this? A bit of feedback would be appreciated!