Add a (setf actions) method to mirror actions

[Ambrevar]

See discussion in https://github.com/atlas-engineer/nyxt/pull/1371#issuecomment-833504558.

[aartaka]

The idea for alternative prompt API is:

• Remove all the actions-on-* methods/slots.
• Make all sources have a (setf ...) :after methods doing the mark/suggestion/follow action etc.

This would simplify the prompter API dramatically.