renderer/gtk: Refactor selection-actions of color-source.

[aadcg]

Description

From #2555

Checklist:

Everything in this checklist is required for each PR. Please do not approve a PR that does not have all of these items.

• I have pulled from master before submitting this PR
• There are no merge conflicts.
• I've added the new dependencies as:
  • ASDF dependencies,
  • Git submodules,

    cd /path/to/nyxt/checkout
    git submodule add https://gitlab.common-lisp.net/nyxt/py-configparser _build/py-configparser

  • and Guix dependencies.
• My code follows the style guidelines for Common Lisp code. See:
  • Norvig & Pitman's Tutorial on Good Lisp Programming Style (PDF)
  • Google Common Lisp Style Guide
• I have performed a self-review of my own code.
• My code has been reviewed by at least one peer. (The peer review to approve a PR counts. The reviewer must download and test the code.)
• Documentation:
  • All my code has docstrings and :documentations written in the aforementioned style. (It's OK to skip the docstring for really trivial parts.)
  • I have updated the existing documentation to match my changes.
  • I have commented my code in hard-to-understand areas.
  • I have updated the changelog.lisp with my changes if it's anything user-facing (new features, important bug fix, compatibility breakage).
  • I have added a migration.lisp entry for all compatibility-breaking changes.
• Compilation and tests:
  • My changes generate no new warnings.
  • I have added tests that prove my fix is effective or that my feature works. (If possible.)
  • New and existing unit tests pass locally with my changes.

[Ambrevar]

@aartaka You confirm?

[aartaka]

I confirm (=^･^=)

[aadcg]

Merged via cf49753.