obs-frontend-api,UI,docs: Add API to add custom docks with no toggle

Some plugin does that by deleting the QAction returned by
obs_frontend_add_dock().

Now that obs_frontend_add_dock() is deprecated,
obs_frontend_add_custom_qdock() replace this usage.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f