Refactor shell theming module (#806)

## Description

This PR refactors a few operations to make the code handle edge cases
efficiently with marginal performance improvements.

## Type of change

- [ ] Bugfix (Change which fixes an issue)
- [ ] New feature (Change which adds new functionality)
- [x] Enhancement (Change which slightly improves existing code)
- [ ] Breaking change (This change will introduce incompatibility with
existing functionality)

## Changelog <!-- This is optional, but highly appreciated. -->

- Stores the comma separated string representation of valid versions for
later use in raising exceptions.
- Uses tuple for `async_data` in `ShellTheme` since its members are
immutable.
- Uses tuple decomposition to assign theme variant and preset from
`async_data` later.
- Replaces `Gio.File.make_directory_with_parents` with Python's builtin
`os.makedirs` with `exist_ok` as True to implicitly create directories
when nonexistent.
- Instead of relying on `KeyError`s when accessing dictionary values,
the `get` method is used to supply a default value.
- When a key might exist in one dictionary, the `get` method (which may
return `None`) is coupled with the nullish `or` operator to use the
value that is not `None`.
- Uses predefined `main_source` attribute as argument to the
`_compile_sass` function.
- Uses the walrus operator to scope a template match to the condition
when it is not `None`.
- Removes unnecessary implicit `.close()`s on file handles when using
context managers.

## Testing

- [x] I have tested my changes and verified that they work as expected
<!-- Make sure you did this step before marking your PR as ready for
merge. -->

### How to test the changes

<!-- Optional, it can speed up review process if you provide the
information on how to test your changes. -->
Just run `./local.sh`

chore: update some paths to contain information about 0.4.1 release

translate: update translation files

Updated by "Update PO files to match POT (msgmerge)" hook in Weblate.

Translation: Gradience/Gradience
Translate-URL: https://hosted.weblate.org/projects/GradienceTeam/gradience/

translate: update translation files

Updated by "Update PO files to match POT (msgmerge)" hook in Weblate.

Translation: Gradience/Gradience
Translate-URL: https://hosted.weblate.org/projects/GradienceTeam/gradience/

feat: fix name

feat: update release notes (#659)

# 0.3.2 release notes

Description of what changed in this release

## Changelog

- Added 0.3.2 release notes

## Screenshot

If applicable

ci: generate .pot file

fix: preset directory issues (#494)

Written on github's editor, and not tested yet 😅 !

fix: disable plugins

release: 0.2.1 (#296)

Please tag 0.2.1 afterward.