This is mainly just to reduce the amount of files in the project root.
The criteria of what gets into `etc/` are the following:
* The file should not be necessary for building nsxiv. This excludes the
`icon/*` stuff since that's needed by `window.c`.
* The file shouldn't have any valid reason to stay in the project root.
This excludes things like `README.md`, `.gitignore` etc.
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/350
Reviewed-by: explosion-mental <explosion-mental@noreply.codeberg.org>
