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>
* includes are sorted alphabetically
* their grouping and layout is the following:
- nsxiv.h will be the first include
- followed by any internal headers (e.g "commands.h" "config.h")
- followed by system headers (<stdlib.h> etc)
- followed by third party headers (X.h libwebp etc)
* also add `llvm-include-order` check to clang-tidy so that it can catch
unsorted includes during CI.
