Prepare v1.3.0 Release

Prepare v1.2.1 Release

Prepare v1.2.0 Release

Prepare v1.1.1 Release (#40)

Prepare v1.1.0 Release

Prepare v1.0.0 Release (#12)

- Adjust "Release" section in contributing guide.
- Disable "X" datasource, since it is not working after the latest API
  adjustments.
- Update "version" key and "msix_config.msix_version" key in
  "pubspec.yaml" file.
- Change user in "AddSourceReddit" widget help text.
- Enable macOS App Sandbox.
- Adjust logo for Windows version.