Extract more GDK::Command classes

What does this Merge Request do?

Refactor existing hard-coded GDK commands in GDK root module, extracting them to GDK::Command classes

This will provide us with a path to later refactor that big case/when into something much smaller and standardize how the commands behave, are invoked and return data/status information.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.