Standardize commands interface in GDK::Commands

What does this Merge Request do and why?

This is a big refactor on our commands implementation to standardize them with a similar interface, and removing some of the mess in ::GDK root module.

I've also added many missing specs and did some small refactors and cleanups everywhere related to commands.

To be easier to review the changes, I've separate step by step the refactor and changes in their own commit, so start from the first commit when reviewing.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.