GTK forces flag images to a square pixel size and scales width/height
independently, so flags get stretched to a square instead of preserving
their aspect ratio.

This change loads flags into a square bounding box and letterbox them
onto a transparent square canvas before passing to GTK, so the
distortion never happens in the first place.

Fixes #11

Bumps [actions/checkout](https://github.com/actions/checkout) from 6 to 7.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v6...v7)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Replace Travis CI configuration with GitHub Actions workflows.
Add CI build workflow for Debian, Fedora, Ubuntu, and Archlinux.
Add release workflow for automated GitHub releases on tags.
Add dependabot configuration for GHA pin updates.
Remove obsolete .travis.yml.

Same as https://github.com/mate-desktop/mate-wayland-session/commit/9bc03ecd9cfdb7ba16a76fa6a569982222d3d422.

Since we install the org.mate.peripherals-keyboard-xkb gschemas here,
it makes sense for us to do that. This is also done in autotools build via
@GSETTINGS_RULES@.