qt: add kvantum theme options

[mikaeladev]

Description

adds qt.kvantum.theme.{name,package} options, closes #3641

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
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• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

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  • Basic tests added. See Tests

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